Letting in the RR rear pipe is all about 'attitude'.
Not YOUR attitude, but the tilt, yaw and roll of the pipe itself. I take care of the tilt as I let it in by slipping in a snug piece of dowel into the pipe, about 8" long. This tells me that I'm letting in level and straight with the RR groove... it's instantly visible if it's out of kilter. When I think I have the right depth, I slip a loose rod in the pipe to see if it will go thru the pipe and enter the drilled hole OK.
The skirt of the pipe will determine the shape of the grip, how much wood is on the forestock between the pipe and lock, and the shape of it. SInce the pipe is thin, it is often easy to bend it out of shape accidentally while inletting. It can also bend left or right. You must keep an eye on this bugger as you progress toward home position.
After ten or twenty of these inlets, you'll get more comfortable with it. But I have to say that after many years, it is still one of the toughest parts to let in properly in my opinion.
You're not alone in your frustration.
Tom