So, it's extremely simple. All open cases divided by the fastest processing monthly speed.
E.g., I'm a mid-May filer, and the first wave of cases is about 11K ahead of me.
Looking at Q1 2023 USCIS data for support, I thus establish:
Q1:8509
4M:11345
4.1M:12054
4.2M:12763
4.3M:13472
5M:14182
Q2:17018
I.e., it takes a quarter to touch 8509 cases, 4 months - 11,345, 4 months and a week -12,054, etc.
A concern though is that the approval trough is so wide. So if one isn't lucky (wave 1, 4 months out), they may be in the rearguard, 6+ months out.