When it comes to a matt clear coat, it's really a question of preference. Most enamel-type matt coats can/will eventually start to yellow, and many are not a full matt, but nearer a semi-matt, even after much stirring.
Having experienced these 'problems' for 40+ years, I've recently changed tactics, and now use Johnson's 'Klear' (now known as Johnson's PLEDGE Klear), with the addition of Tamiya Flat Base. It takes a bit of trial and error to achieve a full, true matt finish, but this can be varied, by adding or subtracting in the edition of the flat base, to achieve anything from semi-gloss, through semi-matt to full matt.
Beware though, as, if too much flat base is added, when dry, it will show a white or cloudy deposit in areas. However, this can be rectified by re-coating with the 'correct' mix. To gain the latter, just add a small touch of the 'Klear', test the finish, and adjust as neccessary.
I'm afraid it's not really possible to provide an accurate mixing ratio, but start with the addition of a very small amount of flat base,(normally around 10% flat base) testing as you go. As the varnish dries very quickly when brushed, use a paint brush for the tests/trials, painting a small amount onto a piece of scrap plastic, such as a piece of sprue. Once sayisfied with the required results, use in the airbrush as normal.
Hope this helps.
Terry.