20 is my cut-off, but just watch him as I know you already do. When it comes to axolotls, the cooler the better I think. 16-19 is still my optimum that I shoot for because some of mine stop eating at higher temps which goes against some of what I have read out here too...Higher temps should = higher metabolism. In my case it = unhappy, stressed axolotls.
I know it' not ideal but the lowest mine gets is 19, during the day with the fan on we're talking 20.5 - 21, 22 is when my alarm sounds and I do a water change to get it down in a gradual fashion. his tank would never be at 22 for more than 15-30 minutes though before I dealt with it. He spends 90% of the time between 19.5 and 20.5 and I maintain that with just a little 2x aquarium fan.
Fred was raised in a conservatory though, so I can only imagine that for 9 months he was subjected to heat of anywhere up to 40 degrees, so hot and in direct light in fact that his tank was dark green with algea (i must stress this wasn't me, previous owner).
I have spent a lot of time panicking over temperature, but at 22 Fred's behaviour is exactly the same as at 19, he may just be ever so slightly more lethargic but he still swims, eats and everything.
Ideally I will set up a cooler so that i can keep him at 17 or whatever, but right now he doesn't really seem bothered with 19.5 - 21 so if you can get it sub 21 i really wouldn't worry too much.
Some people though really are very particular with temps and their limits are very different. tbh there are no hard and fast rules as each axie is different, just get him as low a temp as you can afford and bare to and keep good observations. Good tank husbandry is the key when you are at higher temps as if you can keep water parameters good, you will have less issues with disease and fungus (or hopefully none at all).