Cool idea on the reamer. I’m familiar with them, but hadn’t thought of using them to take a hole to final diameter for this purpose. Neat idea!
Indeed, I think I did have an x/y perpendicularity issue, which I [mostly] fixed with some shimming. Looks like we engaged on that thread too 
That said, the evidence there was a stair step that changed along the y direction of the piece. The step on my pieces now is consistent along the piece, so I think it’s just about xy repeatability/locating, not angular deviation.