Cutting down a steerer tube...

Axle-crown could be different as well as headtube angle and the fork rake... you could do the trigonometry to work out the exact position but that's still only relative to the front wheel axle position - any difference in stack & reach of the new frame vs. old would make a difference to where you want the bars (the required stem length might not even be the same).

Much simpler to build it up, make sure the contact point measurements are correct and then trim the steerer once you're absolutely sure it's right.

Gotcha. Much abliged.