None, the second you start buffing with anything it will gloss up a flat finish.

I'm going to ask the obvious, but when you add the M148 to the clear, and you're talking about a 1:1 ratio, you do mean 1 part of M148 to one part of MIXED clear right? In other words, you're still mixing the clear with hardener right? 1:1 ratio is way to much M148 in there anyway and probably a huge part of your problems.

The M148 should be mixed as follows:
It's the same whether using MC260 quick clear or MC161. Both use the MH167 hardener (fast)

Flat Add 51 oz./RTS gal. (128 oz)
Eggshell Add 45 oz./RTS gal.
Semi-gloss Add 42 oz./RTS gal.

So in other words, if you want a dead flat finish, the correct ratio is 51oz of M148 mixed into one mixed gallon of clear (clear and hardener 'ready to spray')

5 - 10 minutes between coats, will be dust free in about 30 to 45 minutes, tack free in 2-3 hours, and ready to tape in 9 hours.

I haven't had great luck with using the M148 as a flattening agent in clear though. It's true use is a metallic control in base. Added to the metallic base coat paint it will affect the orientation of the metal flake. As I recall that is it's true intended purpose, the above mixing ratio's are from PPG though for use as a flattening agent in clear, just relaying my own experience with it.

Also, with the flattening agent in paints, the tip size of your gun and your air pressure will play a huge role in how well it goes on. 1.3mm tip with 29psi is what you should be using.