Examples from various paint companies to flatten out clear

Lesonal Ez Matte Clear

Ez Matte Clear Combo (see tds below), hardener  and reducer

5 to 2 to 3

Rm,(Basf, Diamont) use

Any Basf Clear,Hardener and SM09

Check ratios below

Excel Hot Rod Clear, Flat or Satin

Just Clear and Hardener

4-1 Satin

6-1 Flat

Washing Flat, Matte, Eggshell, or Satin Clearcoat even has its care to consider.  Here is a great explainer video.