As Tim Hewitt would say butt frequency is not interesting. Look at shaft profile first, shaft weight etc. Butt frequency is for building a frequency CPM slope for a set. You can get a rough idea but a 260 CPM driver of one shaft vs. another 260 CPM driver can play totally different like night and day. Or 240 or 250 etc etc.
That is why Jeff Summitt's DSFI works it's comparing apples to apples. Hence the difference on how FST and FST Pro's play. It's all there in the DSFI. AS, Jack Webb aka Joe Friday would say "just the facts Ma'am."