I forget where I saw this, but a good way to calculate a reasonable approximation of presumtive admit/deny indexes is as follows:

Take the 25% UGPA and 25% LSAT numbers for the school and run those through the index formula, and that is approximately the presumtive deny index.

Take the 75% UGPA and 75% LSAT numbers for the school and run those through the index formula, and that is approximately the presumtive admit index.

YMMV, but I did see this on some law school admissions counseling site somewhere.