Its simple math.
If you have a feed at 16.5% protein, and you want something at 20% protein, you need to combine X parts of 16.5 with Y parts of whatever, then divide the whole thing by (X + Y) such that it equals 20.
Written differently (16.5 times X) plus (Protein content of new ingredient times Y) = 20 (X + Y).
and having said that, crude protein should NOT be your only concern, because most readily available high protein products are frequently also high fat sources, so your average energy content of your feed is being changed in unpredicatble ways. Additionally, the amino acid profile of many high protein sources isn't optimal - so you could end up witht he desired crude protein, but end up with much of the improvement wasted for lack of adequate amounts of Methionine and Lysine. There are also anti nutritional concerns to factor in, often tannins, certain fibers, lignins most frequently.
Better, MUCH better, to simply find a higher quality feed more closely matching your desires than to pretend you are a poulry nutritionist and try to "fix" a feed made (in theory) by someone who knows more on the subject than you do.