Pipe threads are to be tapped to a precise depth. You never just run the tap all the way through. (I assume that you know that, but for others that might not)
Without a gage to know where the perfect depth is, I would try to have approximately 4 threads or approximately 2/3's of available threads of the fitting threaded into the tapped hole.
So you need to tap some, then check the thread engagement and repeat as needed.
Brett