You're making this a whole lot harder than it is. Nobody ever believes me when I say this but the only thing that matters is the pivot to spindle distance, and even getting that exactly right isn't super critical. So since no one believes me maybe they'll believe Michael Fremer? https://www.analogplanet.com/content/tone-arm-geometry-101
The relevant paragraph from, again, Michael Fremer:
For the same reason the hole isn't all that critical either. If the hole is a little big, no problem, arm mounting flange covers it up anyway.
If you just can't get over the worry you can always practice on a cheap piece of MDF. But seriously, its not that hard.
The relevant paragraph from, again, Michael Fremer:
Fortunately, there is some room to compensate for P2S distance error here in the form of head shell slots, which are necessary since the distance from the cartridge fixing screws to the stylus tip has never been standardized (this doesn’t apply to SME arms where the head shell holes are not slotted but instead the arm pivot slides).What this means, in practical terms, is you can drill your hole anywhere you want, and as long as the pivot to spindle distance is pretty close it won't matter, because that will be fine-tuned anyway when you do your cartridge overhang adjustment.
Read more at https://www.analogplanet.com/content/tone-arm-geometry-101#QvyoQh0VqBG91c0B.99
For the same reason the hole isn't all that critical either. If the hole is a little big, no problem, arm mounting flange covers it up anyway.
If you just can't get over the worry you can always practice on a cheap piece of MDF. But seriously, its not that hard.