I agree that bio­di­ver­sity off­sets are a chal­leng­ing tool both in gen­eral and under the scope of the Ger­man Impact Mit­i­ga­tion Reg­u­la­tion (“Ein­griff­s­regelung”), they are far from per­fec­tion (and will never reach it). As you say, the max­i­mum we could achieve is to estab­lish valid­ity in a given con­text. I am not sure if I have a def­i­nite answer for you (prob­a­bly not), but one of the sources I found most use­ful in the Ger­man con­text (and in Ger­man) is the dis­ser­ta­tion by Elke Bruns that is avail­able here: https://depositonce.tu-berlin.de/bitstream/11303/1906/1/Dokument_16.pdf (Bew­er­tungs– und Bilanzierungsmeth­o­den in der Ein­griff­s­regelung). If you are inter­ested, I could also rec­om­mend a few inter­na­tional exam­ples (e.g. the Aus­tralian habi­tat hectares approach, the _Defra method­ol­ogy devel­oped in the UK as well, the French guide­lines etc.) — but note that most of them are still under­go­ing some refinement…