Re: band gap accuracy ( No.1 ) 
 Date: 2021/12/27 16:06
 Name: Pavel Ondracka <pavel.ondracka@email.cz>
 This is highly material dependent and could be pretty much anything. My experience is that standard GGAs like PBE underestimate the quasiparticle band gap by ~50%. So in this regard the number of decimal places is IMO irrelevant. If you are instead asking about the precision of the calculations than this depends on your numerical settings, i.e., kpoint density, cutoff energy, basis set, etc.
See for example the Figure 1 in "K. Lejaeghere et al., Reproducibility in density functional theory calculations of solids, Science (80. ). 351, aad3000 (2016)" for the difference between the accuracy and precision and to understand why you question doesn't make much sense.

