There is no simple answer. First of all, the hob must be larger by amount of tip clearance, so it can cut deeper than the worm will ever reach. At this stage the geometry of the hob and the worm are still similar. The hob will probably be sharpened and it will reduce its diameter, so it needs to be accounted for. Unfortunately, this will change the geometry of the hob, e.g. the helical angle, and influence the contact pattern. At this stage it can be interesting, what type of worm will be used with the worm wheel – ZI, ZK, ZA or other. If you have this information, you could calculate the contact pattern of your drive and play with some parameters to see how they influence the meshing. You would probably find, that increasing the diameter of the hob just a little more is beneficial as it makes the contact pattern more stable throughout the life of the hob. This “little more” depends on many factors, e.g. the relief angle, worm type, enveloping angle, material selection, the previous steps.

The clearance and sharpening corrections/enlargements are basic and needed to have any chance of producing a good worm gear, all after that is like optimization.