I did not write the package was from the’70-ies, just that it looked it was designed in that era. Your case outline has a date of 2016-12-20 which is not very old.
The 0.36 is a tolerance, not a dimension. It shows how straight the pins are related to the referance surfaces A and B. I’m not sure what the M and the L in the circles mean, but I am sure they conform to some norm and can be looked up.
The 0.12 in the right top corner is another tolerance and is for the flatness of the mounting surface [B].
Forming of the leads is an art in itself. For high volume production you want to completely automate this, and there are machines that do this. Some even can add S-bends in the leads to allow for thermal expansion.
You can get to a sensible value with a bit of common sense and calculating backwards.
Assume you have a PCB of the normal 1.6mm thick and you want the tips of the leads to stick out 1mm.
This gives the place for the bend 1 + 1.6 + 3.81 = 6.41 mm from the underside of the leads.
The distance from the center of the mounting hole to the center of the pin holes will then be approx.
.>>> case = (17.4 + 16.89)/2
.>>> lead = 13 - 6.41
.>>> case + lead
You do not want to bend the leads too close to the body of the package to prevent bending stress. More standard pins are made from stamped sheet metal molded into the plastic body, but your TO-254AA looks like the pins are made from wires and there is a glass or ceramic isolation ring which may get damaged under bending stress from the leads.
So if you want to bend the leads yourself, you have a hole to hole distance between approx. 21 and 24mm. If it is intended for high volume production you should contact the factory which will make the boards before making a final decision.
Also, for the final size of your holes, there are probably norms about tolerances and such. If you want to keep it simple, then compare KiCad’s library symbol for both TO-220 and TO-247 and compare the hole size with the pin size for these packages. Your TO-254 is somewhere in between those. A coarse guess would be between 1.4 and 1.7mm.