When should i use a flat finish, before or after weathering? What ratio should i mix it at as well?

this is what i do i would do a wash, then dry brush,
then do a flat finish,then after the flat finish,
add your pastels.
i use polly scale flat coat i brush it on.
its the best flat coat u can buy

I guess that you are talking about armour? If that is the case then what I have to say on the subject may not be the way to go, as I model aircraft. Having said that though I might just give you some useful ideas
This is what I do (mostly, as it can depend somewhat on the effect I'm trying to acheive):
1. Apply paint. (Air brushed over a primer coat of white auto plastic primer).
2. Apply gloss coat (I hand brush 1 or 2 coats of Johnsons Klear (Future, in the US). This is to give a smooth coat on which to apply the decals).
3. Apply the decals. (I use setting solutions)
4. Apply another 1 or 2 coats of gloss (see 2 above, these coats protect the paint from the thinners in the washes, in the next stage. These coats also help to level the decals off, removing the step (the thickness of the decal) so that the wash wont highlight it).
5. Apply oil washes and add oil, and dirt streaks. (Washes will flow better on a gloss surface, it is also alot easier to remove mistakes).
6. Apply flat coat. ( The thinning ratio of you flat coat depends on the make, start with a 50/50 mix and take it from there.
7. Apply any dry brushing (I rarely do this on the exterior of a plane model. Dry brushing will work better on a matt surface because of the "tooth" it has).
8. Apply paint chipping (I use a silver pencil, again the "tooth" in the matt finish makes this much eaiser to apply. In fact you wouldn't be able to use this method on a gloss surface).
10. apply gun and exhaust staining. (This is applied last because in real life this staining would be on top).
Although I don't model armour, the basics of what I say hold true, regardless of the application.
HTH
Mal

I do the following: gloss coat, decals, gloss coat, FLAT, then weathering - washes - etc.
When all is finished, there is a very, very slight sheen to the model(except where the pastels are).
capnjock

I usually flat cote after most of the weathering. Only thing left to do is dust coat with tamiya which dries very flat and pastels. Anything that needs a slight shine after that, I go back with satin cote or silvering by paint or pencil!