Another part of tramming is to extend the quill; that doesn't change the angularity, just magnifies it. When you retract to mill, the error will be lessened.
When I tram, it's over well matched 1-2-3 blocks. It's then easier to swing a larger circle and X or Y for the difference. On Bridgeport pattern mills, do the 'nod' or Y axis first, then X, and recheck Y.
The only instrument I use; Indicol clamp, Interapid test indicator. If it needs to 'perfect', I extend the clamp/ indicator far as they will go.