Yeah, total hook ratings have always been a mystery to me; for both of those sources.  BTM at least acknowledges it is probably the least relevant part of their ratings.  As such I pay much more attention to length/torque/backend as well as tweener ratings on the different conditions. 

To this day I have no way of interpreting Perfect Scale.  There is zero explanation of how it is derived or what the number actually represents; other than the supposed relativity to other balls' Perfect Scale rating.  It may be the single most useless 'rating' I have come across to date.  My opinion.

I've been comparing BTM vs. BJI numbers for years.
My basic conclusion is that numbers are good ONLY within any one system, if that. Comparing BTM to BB.com or BJI to BTM is fruitless and fraught with peril. You're much better off trying to extract information based on the words used to describe the ball than the numbers generated by any one source.

Look at the reasons behind the numbers:
- what kind of bowler is doing the testing for what purpose?
- how is the ball drilled? For generic use, for maximum hook, for maximum backend, for minimum hook?
- what surface did they test in what oil patterns, to what end?

Many times, the testers don't even know what the ball is designed to do.

BTM puts the best (not necessarily the strongest) drilling for each of their testers: stroker, tweener, cranker. What BB.com does I don't know.

BJI puts a generic drilling on theirs.