Is it better to consider LTFT and STFT together?

correction = (LTFT B1 + LTFT B2)/2 + (STFT B1 + STFT B2)/2

This assuming that x% correction for STFT has the same weight for LTFT also.

Example: +4% LTFT and -7% STFT means a total adjustment of -3% (for a particular FTC)