Welcome to the forum. It sounds like more of a technique issue rather than the paint you are using. The paint mix seems OK but is not a steadfast rule. I only use MM enamels and have found that they vary in thickness out of the bottle. So ratios will change. The trick is to get that milk like consistency, what ever the ratio.
What kind of tape are you using and how many coats of paint are you putting down f to get the full coverage you are after? The thinner the tape the better... even if you still get the line. Thick tape=tall ridge, thin tape=shorter ridge. For a hard edge paint mask I always use Tamiya tape. If I am worried about it lifting the undercoat I will stick it to the back of my hand first to get rid of some of the tack. If you are putting down too much paint at once it has more of a chance to build up along the tape. Spray several very light coats and build up the color. Unless you are doing a tight camo pattern and the tape edge are all close together, angle the spray a little from the tape side of the line, spraying away form and not directly into the tape edge. At the very least keep it 90 degrees tot eh surface.
And if you do get the occasional ridge at that point when the paint is tack free and not 100% cured, still a tiny bit soft, I will carefully roll a paint brush handle over the ridge and most times it will flatten out pretty well.