The age-old question.  One of the primary arguments in favor of brand loyalty (without getting into that debate).

Basically, it's very difficult.  But, if you can get a single pair comparison to work (say find two balls that are comparable), then you can guesstimate from there.  Turns out the Big Time is comparable to the Bully (who knows?  Not me).  Well, the BT has a hook potential of 28.75, the Bully 53.  So if the Ice has half the Ebonite hook potential (guess), and the Jeckyl has half the hook potential of the Bully (another guess), then maybe those two balls are similar as well.

That will give you a rough idea of what's similar.  Of course, you have to also factor in things like surface finish.  It won't do to compare the 500-grit EPX-T1 to the 4000-grit polished The One, even if, based on an argument like the above, they are otherwise similar.

At best, you can kinda group them and order them within a single company.  The XXXcel, XXcel, Xcel..., Action, Bully, Backyard Bully,...  Sort them from heavy oil to light oil, and guess that the heavy oil balls from multiple companies will handle roughly the same amount of oil.  That's not always the case, but usually close.  Then work your way down the list.

Or, the much more reliable method, ask someone with both balls how the two compare.  Or normalize the numbers.  If Brunswick's scale is 0-175 and Ebonite's is 0-50, then a 30 on Ebonite's scale is about 105 on Brunswick's.  Maybe.

SH