Hi Steve,
I find the offset calculation a bit of a crapshoot at slow focal ratios as you suggest. I do FMax focus runs on each filter in turn and toss any result that finishes with a HFD that looks out of whack. I do this several times and put this all in a spreadsheet and calculate the offsets for each run, delete any outliers, then average the offsets. Then I use a Baht mask and Bahtgrabber to check and tweak the results. This process takes a while but fortunately I don't have to do it often
Cheers,
Rick.