We've probably all been there. Certainly those of us who were building before internet modeling sites. The real problem with this garbage is that it's not properly labeled. The label and Tamiya's site had instructions for its use. They were the same,. word for word, as the instructions for their paint, in essence, apply, wait a while and apply some more, if needed. Not a word about it being an additive. Even then, the comcept in most model;ing applications makes no sense. If you have a flat finish, you need to make it glossy again to apply decals.
I know, there are folks who swear by this stuff mixed with Future. I learned something a long time ago...K.I.S.S., Keep It Simple, Stupid! There are planty of clear flats on the market that don't require exacting mixes to achieve a perfectly flat finish right from the bottle. Need a satin finish? Add clear gloss to the clear flat. Otherwise just use the single, one step product to achieve the flat finish you want.