Obviously just speculation without actual measurement, but how much is par reduced by having floaters? What do you think? Say at 12" just for kicks. 5, 10, 20? If I had a par meter I'd check myself as I think it would be interesting to know.
It depends on what kind of floaters you are using. Obviously, frogbit will block out more light than duckweed. For something like frogbit, if you allow to cover your tank, then I'd say you're easily reducing your light by 75%. For something like duckweed, I'd guess it's closer to 50%.