Re intermediate coats, use Klear straight from the bottle.
You only need to flat your final coat if you want a satin or matte finish.
I have a jar of matte Klear in my man cave I mixed oooooh, 8 months ago with Tamiya flat base, give it a good shake and it's good to go.
One cautionary note with flat base, mix it well, I mean, stir it, shake it till you think its done then do it again for another 3 minutes, if it's not thoroughly mixed you'll get cloudy blotches, it's not an ideal solution, i'd personally recommend you use straight Klear and get a satin and a matter top coat varnish.
Ratios however if you want to go that route, use a syringe, be accurate.
10% flat base = Semi gloss
20% = Satin
30% = Flat
Don't go greater than 30, you'll get the white coudy thing.
These are ratios I found through googling then trial and error, they work for me, I use little medicine cups and a small syringe to decant the flat base to ensure the ratios are correct.
For applying, AB is OK but with it being so thin and watery i found it easy to get runs, easiest and quickest method and the one I use all the time now, is, wide flat brush and brush the whole model quickly top to bottom, Klear is so thin it self levels you can't get brush marks, make a mess just brush over with Klear again, it dissolves in itself.
Alternatively, if you have a container large enough you can dip the whole model, let it drip then sit under cover with some kitchen towel to wick away the excess.