High slope is calculated from injector flow, ( lb flow/3600 ) to get in the ballpark, but the 58lb rating does not seem too accurate. The low slope and breakpoints are very hardware specific, but you can use the adaptive fuel terms of the, what is commonly called short term fuel trim, and the long term fuel trim to dial it in.
Typically the idle adaptive (long term fuel trim, lowest air mass) will show if the low slope may not be correct. If the other (higher air mass) adaptive values are close to nominal, most likely the high slope is pretty close, leaving the low slope to correct the idle adaptive cell, provided you do not have an air leak.
If you don't have either the MAF transfer function or the injectors accurately characterized, you really have no way of knowing which, if either is correct.
I'm sure someone has a 3" maf transfer function out there somewhere....