#8 shot will serve you well at the 20 yd line, and a lot farther back than that. Shooting from the 20 is the same as from the 16. No changes are needed until you get back around the 24. Shoot those #8s.
Once you get back to 20 yards, you should shoot Max Dram High Brass #6 as you are a lot farther back than 16. About a 5 foot lead on the hard angle targets too.
In seriousness, 8's are fine. You should see no difference at that short of a difference from 16. I have been told you are good back to 23 with what ever you use at 16 so 2-3/4 dram 8's are fine. 24-27 you may want to stiffen up some like a 3 dram load. I know guys who still shoot 8's back at 24-27 but I think 7-1/2 are preferred by most guys back that far.
gjmen.....Only you can answer that question. Here is how. Take 2 boxes of the 8's and 2 boxes of the same load in 71/2's and shoot your handicap with them. After you are done, compare your scores with both and your answer will be obvious to you. Both will break targets from anywhere on the pad, however, some barrels pattern 71/2's better than 8's, and vice versa. Determine which your barrel likes best and go with them. My DB-81 just loves 71/2's but won't shoot 8's worth a darn for handicap........Just my experience....Dan Thome (Trap2)
To answer your second question, #8 starts to fall the minute it leaves the barrel. As a matter of phsyics, if you shoot a load of #8 perfectly horozontally, and drop a load of #8 from the same barrel height, they both hit the ground at the same time.
That can be said only if you shoot your gun perfectly level, not pointed up or down at all, also the ground must be at the same level both where you stand and where the shot will fall. Tilting the gun up a little will greatly increase the time the shot will be in flight. -Rich