Ok I know most recipes call for a 16oz mini bag. Well they just don't sell 16oz. mini bags around me. Only 10.5 oz. So so how much should I be using? A 10.5 oz bag plus how much more?

If you have a kitchen scale, you can weigh out 16 oz. Otherwise, a bag and a half of the 10.5 oz size is pretty close, close enough to work.

I buy a 1.36 kg (about 3 lbs) bag in Canada. I can get three batches of MMF from a bag. Each batch fills an 8 cup measuring cup. Now I don't weigh my marshmallows I just use my 8 cup measure.

I use 3 10.5 ounce bags and make a double batch (1/2 ounce shy of 32 ounces is close enough for me)

Thanks!!