ἀβασκάντως τὰ ἄριστα πράττων is incorporated into the regular sign-off, as a superstitious coda. ἀβασκάντως is something like “without incurring the evil eye.” The evil eye was quite a thing, and not just in Roman Egypt.
I once had an interesting literary papyrus that accused Aristotle of βασκανία against Plato, perhaps the most extreme example of the Platonist/Aristotelian divide.
πράττων nominative gets me a little. I’d think it the same sentiments of “ευ πραττειν”, whether or not he believed in literal witches or malevolent stars. But with nominative πράττων, isn’t he saying, as written, “above all else, I pray you to have good health as I work the best without ill-enchantment”? Or is it formulaic and implying a new phrase?
Your translation gives the literal meaning pretty well, but it’s a fairly colorless phrase, conveying good wishes. Cf. e.g. POxy.300, a personal letter from a woman which towards the end has ασπαζω Θεωνα τον κυριον και (4 more names) τους αβασκαντους—i.e. protected from the evil eye from malicious ill-wishers, but the sense is much faded.