I switched to BIAB on the stovetop a few batches ago, and I generally get a brewhouse efficiency between 55-65%, so I build my recipes at 58%. I use a 5 gallon paint strainer bag from the hardware store, stir and check the temp every 10 minutes after doughing in, and check the gravity after 30 minutes. If it matches my target preboil gravity, I'll move onto the boil, but if the points are still low, then I'll wait for the full hour, still checking and stirring every 10 mins. Oh, and after a great tip from
@Herm_brews, I take the top rack out of my oven, set it to 170F, and leave my mash in there between stirrings. Yesterday I stayed at 152F for the whole mash for the first time.
Edited to add: I buy all of my grain milled, sometimes from my brick and mortar LHBS, sometimes from online retailers. I recently ordered my grain double milled, but was disappointed I didn't see a substantial increase to extract efficiency. Maybe as I reach the end of the bags and more of the grain flour is added, that will make more of a difference.