I've been trying some favorite baking recipes from my American cookbooks
but I'm now living in France and they don't come out right. Cakes come
out a bit heavy and leaden. I know that French flour is different from
American flour, and I wonder if there's some way to figure out how much
more (or less?) to use. I do have American baking powder, so it's not that.

I think French sugar is coarser than what I'm used to as well. I find it
takes a lot longer to cream butter and sugar if I use standard French
sugar.