Hi I agree. Washes can be maddening. You would think it would be a fairly simple process, but, unfortunately, it is not.
I haven't had much experience with washes, per say. But I am learning. I do not use any fixed ratio color to carrier in my oil washes. Just mixing a little color into a small dimpled paint pallet until it just looks 'right'. A dirty thinner type of solution is the key.
I always give my models a coat of clear gloss varnish before putting on any decals and then seal it again with either Semi- or Dull-cote. So really, it has two coats of sealer in the basic paint job. I hear oil thinners are really hard on model paints so I make sure I do this before using oil washes.
My biggest success so far is in using a small, and old, brush to dip in the wash and putting it in the places that I think it should go, just a little bit at a time. And I use the 'sharp' end of a napkin to dip into the wash if I think I've put on too much, to absorb the excess. Once I get this done I put things aside and let it dry and see how the finished wash looks and then if necessary apply a little more here and there if I think the model needs it.
I have always heard that in any weathering type of application it's better to have too little of soemthing than too much.
Like anything else in this hobby, you get better as you go along and nothing in it, esp. in the painting and weathering departments is simple to learn to do. Just takes lots of patience and practice. Also quite a bit of forethought before dipping brush to paint or washes.
By the way I think you did a good job on your flak 38. Looks good to me.
Take care, sgirty