STUDY YOUR ROSTER FIRST
The first thing the Seahawks do before they set their board is grade their own roster. The accuracy of how they grade their own roster is probably just as important as the accuracy in which they grade draft-eligible players.
Let's take the safety position. Let's say Earl Thomas is graded at an 8.0 and Kam Chancellor is a 7.2 (on a 8.0 scale). You also have Earl and Kam locked up for the next 4-to-5 years. Let's say Jeron Johnson is graded as a 6.1, but you lose club control of him in one year. You are in the market for a good backup. Let's assume that the Seahawks will run a lot of nickel, but not a lot of dime- so the 3rd Safety is probably more a special teamer.
Now, let's say Deone Bucannon is graded as a 7.0 on your board. Do you really need three safeties with 7.0 grades? Though it would be quite a luxury, in reality, he is probably replacing Jeron Johnson, not Kam or Earl.
Now, let's look at the cornerback position. Let's Say Richard Sherman is a 7.8 and Byron Maxwell a 6.9. Call Jeremy Lane a 6.2. Tharold Simon, perhaps a 5.6.
You lose club control of Maxwell in one year, a starting position, and you have a nickel position up for grabs. Let's say there is a corner on the board at 6.95 and imagine Deone Bucannon was still available -- would you truly take the Best Player Available (BPA) for a slight uptick in score when you have Earl Thomas and Kam Chancellor: young, hungry All-Pros?
I think the mistake I make when projecting NFL team needs is they look only at the needs in 2014, while the best NFL teams are looking at the key needs for 2014 and 2015. That, and the fact that outsiders do not know how teams value their backup players. Sometimes the teams know that these "no-name" players have talent, and have inside information that reveals that they could start in the near future, and perhaps excel.
I believe the grade of your existing players, the minimum number of players you feel you need with high grades in each position, and club control will determine the position groups you target at different areas in the Draft. I think it is acceptable to select lower graded player by a smidgeon: Taking a 6.95 player in a position of need instead of a 7.0 player in a position of non-need is not a sin in my book. However, taking a 6.4 player in a position of need instead of a 7.0 player sans need probably is a sin...
POSITION BUCKETS DOWN THE CHAIN
Let's say there are 11 position groups for your team, and in an ideal world, your team has no gaping holes in any group. Some NFL teams do have gaping holes, and all NFL teams have holes of some sort. No matter how strong or how weak your roster is, you would then likely target three or four positions of top need and focus on those in each area of the Draft where you hope to select.
You then target three or four key position groups when your pick arrives and perhaps you have five players you would be happy with -- the highest graded players in your top three or four needs. You would then rank those five players and be very clear in your mind who you would take if all five were still alive. (rhyme)
Once you have made your selection, you will likely not pick that position group with your next pick (unless you have serious holes). If your roster is built well, you probably have only one opening in that position group, at least for early in the Draft.
John Harbaugh, before the Draft, said something to the effect of, "If we take a corner, then we are probably not in the corner business for the rest of the Draft".
People do draft twice in the same position group back to back, but probably on rebuilding clubs. I would imagine that is more rare for a "mature/good" ballclub unless another tremendous talent is falling.
For your next pick, you then still have the other three position groups that you have yet to fill, and then you add another position group to the mix.
Perhaps a team's top needs are a QB, WR, RB, and DE. Let's say they select a QB with their first pick. When their second pick comes around they may now say, "Our top 4 needs are now WR, RB, DE and CB". Perhaps this chain is repeated down the draft. Once you hit the 6th Round, perhaps you start swinging hard to 100% NEED-based or 100% BPA-based drafting.
At that points you are filling remaining holes and hoping it works -- or perhaps taking a flyer on a high upside athlete.
I do not believe a team has completely preordained any sort of trade-back. If you are ten picks away from your slot and you have your top-five players in perhaps four areas of need, you would trade back only if
(1) They were all gone or
(2) Most of them were left.
If all five were left and very similar in grade, and you could slide back 4 slots, you probably take that slide back. If only one or two were alive, you should probably just select your man and move on.
Seattle is probably calling every team from 41-49, and the call goes something like this:
"Hey, we have a handful of guys we want at 40, but if they are all gone or all of them start to slide, we are willing to move back to your spot pending mutual acceptance of compensation. Are you open to moving up or are you likely happy to stay put?"
You would try to feel them out with no promises either way, and just try to gauge their willingness or reticence.
Determining your compensation may start with a Jimmy Johnson chart, or perhaps an "AV" chart, but in the end I think two things will rule the day in terms of compensation:
(1) Supply and demand, ideally you have two teams bidding for your slot and you can drive up the price.
(2) The "Talent Curve" of the Draft in that particular year -- it may be fat or skinny in Round 4, and that could make teams more willing or more reluctant to part with a 3rd, 4th, or 5th Rounder in order to move up.
In a related thought, obviously any sort of "cliff" in terms of a drop-off in player value will motivate a team to move up.
The Vikings thought that there was a QB cliff after Bridgewater, they probably believed Teddy's grade had a good size gap over Derek Carr.
The New Orleans Saints traded up because they thought that, after Brandin Cooks, there was a big gap to the next WR for the role they had envisioned.
Philly lost their man, Ha-Ha Clinton-Dix, as he went to Green Bay, so they were willing to move back and then take DE Marcus Smith.
Keep these in mind when Seattle goes on the Clock at 40 and 64.