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Expect thin and potentially patchy coverage when thinning 1:1, let alone diluting beyond that point. Dilute paints always require multiple coats for full, even coverage - even "foundation" or "heavy" versions with higher pigmentation.
The only real issue you seem to be having is with paint not staying where you put it. Additives can help you, here, but they shouldn't be considered necessary. You say you're wiping your brush off before going to the model, but how much paint remains in the bristles? There's a certain "critical mass" necessary for paint to be able to pool and run, and it sounds like you're still overloading your brush for your current level of dilution. Even pure water will stay where you put it, if the brush is merely damp.
Try loading your brush even more lightly and wicking away nearly all of the paint - you won't need to scrub, just use some paper towel and let capillary action do its thing. It may look like all of the paint is gone and your bristles are merely stained, but there is still paint hidden in the belly - lay a few strokes down and you'll see color transfer to your model. This is the only way I've been able to apply highly dilute paint to areas with high texture and sharp edges; on larger surfaces, a bit of running is less of an issue, so you can go right to the model after lightly loading your brush, then simply spread the paint around until it's sufficiently thinly distributed to be unable to pool.
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