Putting a Known straight tube that fits snugly inside the steerer and having it pass By the fork tips offers a reference point

Though Butting in the bottom of 1" steerers wont help that be easy .

the Flat table, V block and flat block under the fork hold that fixed

then Lay out an accurate base line on the table that is the same as the centerline of the steerer tube..

height gages use the flat base as a Zero point.. to get the axle CL to the base distance.

you should be able to measure the distance variations on either side of the CL to the fork tip face.


(perhaps buying thick Plywood and making a way to Rigidly fix your fork steerer holding V block will help your Improved accuracy in measuring ..

a DIY alignment table ..