If you are buying your paint from an auto paint store, they will be able to add a flattening agent when they mix your colors. Or they should be able to add it to clear.
Yeah I used flattening agent in the clear on my convert-one quart to a gallon of clear and it really didn't do much to dull it.Maybe a 1 to 1 ratio would work. I bought the stuff at the paint supplyer.
the newer high solids clear coats don't flatten out that good by adding regular flattening agent. most paint manufacturers make pre flattened clear coat just for that reason. i know PPG has 2 different flattened clear coats.
I have also tried to use flatting agent in the regular clear . IT did not work that good you would have to use quite a bit of the stuff to make it flat . i use it 25% on the plastic satin Finnish parts on some of the foreign cars and it works OK for that .