Wish I had some numbers but don't. It's difficult to judge "just forming". If we had the data from a certain size until ripe it might be easier. I think I'll mark a few BM and Preto that are a certain size, say 1/4 inch, and see when they ripen. Now would be the time to set that up.
Does 1/4 inch diameter not length sound like a good starting point? My guess is fruit now 1/4 inch still has a chance to ripen in longer season areas. I'd think in September with constant 90F highs in my greenhouse. And it would be interesting to see if I can really measure a difference between BM and Preto. I think Preto is earlier.
So I marked 4 shoots of BM at the 1/4 inch stage. Preto has some early full sized fruits and lots that are just forming. I've only got one fruit at 1/2 inch. Also marked some other varieties at 1/4 inch stage.