Quote:

IMO, Powley having used his calculator many times to determine velocity for a specific twist ratio for lead bullets I've not shot before

But I do have to say, Don Miller published a black powder calculator adaptation of the Green Hill formula in the BPC News in the Spring of 2007.  What caught my eye was his calculation of 17.65 factor of the Ideal 375166 bullet in a 1:18 twist.  So it took me 2 years for a group buy of the remake of the mold.  I then used the Powley calculator to determine the ideal velocity which when tested was 42grs of BP for a 18 twist barrel.
This is the results of the 1st range test of the 375166 bullet with 42grs of BP ...
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1. If you pm me with your email address, then I'll send you an excel workbook that calculates Greenhill, Dell, Powley and Miller twist rates simultaneously.
2. This workbook says that with your inputs, .321", 1.05", 185 gr, 1500 fps, 16" twist, 
Greenhill twist is 14.7"
Dell twist is 13.3"
Powley s.f. is 2.01
Miller s.f. is 1.44 vs. 1.46
3. Powley and Miller use s.f. in the calculations. With s.f. of 1.5,Powley twist is 18.5"; Miller twist is 15.6"
4. Your question assumes that the 16" twist is correct, and there is clear disagreement amongst these four regarding "correct" twist. At the Powley/iller stage there are a set of 6 major variables, dia, length, wt., mv, s.f. and twist. We must know 5 to know the 6th.
5. I suspect that the difference may lie in the definition of "twist". Greenhill twist is the minimum twist required for bullet stability, and except for very long range with large calibers, it works. There is no recorded instance of Greenhill failing under 600 yards.