The Audax needs 57mm drop calipers. The best in my view are TRP RG957..... but they are quite pricey.
they are nice brakes (and are compatible with NSSLR levers) but (47-57mm) they will be at 'full stretch' when fitted to a lot of audax type framesets. This will result in the caliper MA being significantly lower (up to about 25% lower) than if the brake blocks are in the upper half of the slots. There is a chance that on some frames with mudguard clearance the reach won't quite be long enough.
The R559 calipers (55-73mm) have the opposite problem; they may not be short enough for some frames with mudguard clearance. But if the brake blocks are in the top half of the slot the MA will be as high as it is ever going to be.
FWIW the higher caliper MA with current brake designs has made the
variation in caliper MA which occurs when the brake blocks are set to different depths greater than it ever was before. In fact it is so great that the difference between the top and the bottom of the slot is as much as the difference between generations of lever.
Anyway measuring up carefully will tell you exactly what brake drop is required (IME it can vary slightly even in frames that are meant to be 'the same') and then it is a question of choosing accordingly.
PS. Oh- I just noticed that you are intending to use 7800 levers; these are pre-NSSLR type which means that neither of the calipers discussed above are correct. In shimano, BR-R650, BR-R450, BR-R451 should have the correct MA and about the right reach. Most older DPs from various manufacturers will be in the ball-park too, MA-wise. If they fit then BR-R650 (which is just going NLA right now, but I think you should be able to find them if you look hard) is the best choice; unlike the others BR-R650 has ultegra quality pivots using ball bearings etc .
cheers