My favored approach to offset drilling / drilling on curved surfaces is to use an endmill. Doesn't wander, goes straight in. Of course, you need an endmill for that approach, but id you're building a CNC those should be in ready supply.
interestingly you can also mill a flat or indexes to mate tubes together. I’ve also contemplated making one-piece saddles to mate extrusions, filling the gaps with zero-expansion epoxy.
Tubes and extrusions you buy cheap rarely have dimensioning and tolerance you’d want to accept out of the box. To get what you need, best just to use geometry of hole centers, and adjustable fine parts.
I set a limit of 1/128 inch on any garage woodworking projects. This is 8 mil (thousandths of an inch) or 0.2 mm. Wood and plastics (and even aluminum) fluctuate from moisture and temperature enough to make this a lower limit of reasonable value, though I’m getting closer to 5 mil in router precision. It’s not a fine carpentry shop and I’m not making anything that really needs better than eyeball precision (hand marking) which would be about 1/32 inch.
Applying geometric dimensioning and tolerance to design has been a liberating experience. I’m not a mechanical engineer or even otherwise anywhere close to the industry so I really had no idea how to assess or compare designs.
