When I created some custom terminal blocks, if I remember correctly, I solved the insertion specing problem by carefully selecting the insertion point for the block I created. When I was selecting the center of the block, my block was inserted with a distance to the other terminals (and overlaping with the ones on top). I took note of what that distance was (by creating a line) and when I created the block again, I used that line to offset my center. That did the trick for me.