I have marked , with a fine-point inkpen, two templates from each scale length I utilize. I used factory necks to mark them out and double checked them against guitars they weren't created with to check the accuracy. I've been able to slot FB's since then and haven't had any issues with intonation. I use theguitars to record with so intonation is incredibly important from guitar to guitar and all over the neck. Always double/triple check your slots against a known good scale before commiting to the saw. Pencil marks can be erased , saw marks not so much so.
and to answer your original question :
: "as accurate as you want your notes" .