The Manual of Acarology states 60-95% (I think). 70% works fine for the little work I do on the group. Some workers use 50% glycerol to dilute the lactic acid, which seems like a good idea, and presumably helps keep specimens a bit pliable (so reduces breakage).

I don't know if there is a "best" concentration. A bit like a cake, you cook it until it's done (albeit at 40-50 C), with smaller ones being quicker than larger ones.

Thank you for your help. Maybe I will try with glycerol.