I don't cluster thin until after berry set. I read an article once that said cluster thinning too soon will cause the vine to compensate by making larger berries. That is typically not desired. Plus, weather may impact how well your pollination and berry set goes, and that might impact which clusters you keep and which ones you cull.
I keep 2 clusters on the healthiest shoots that are longer than 2 ft. I keep one cluster on healthy shoots between 1 and 2 feet, and remove everything from weak shoots less than 1 ft. I will let some hyper vigorous varieties have 3 clusters on the strongest shoots, usually in the head region of the vine. But these get spot checked after veraison to make sure they are ripening evenly.
PS: nobody ever told me I was an expert at this.
H