I tend to agree with your assessment on positions from the rail out, although a track like Charlestown has tight turns and a short stretch run after the final turn which is way different than a Belmont. In a 6F race you can calculate the second call fraction by subtracting the first call and adjusting accordingly for lengths behind. Or you can cheat a bit by using Bris E2 pace numbers. Using those numbers and a horses finishing position/lengths behind, you can guesstimate if a horse is improving or declining. Those are my thoughts anyways.

I have been looking at the actual internal fraction around the turn.