I have known folks who use the water weasels and when the temp in the weasel is 99.5, the air temp in the bator can be as high as 102 to maintain the weasel temp
I don't use the weasels (don't trust that high air temp) so this is speculation
I would say if you are doing THAT, then turn it down slightly at least or the eggs will dry out too fast when they are trying to hatch.
trial and error though. I do run my little giants at about 100 or 100.5 when the air around is very cold. This has worked well for me, but I don't lower the temp for the hatch, I just add LOTS of humidity. in the summer time when the smaller bators don't have to work as hard to keep warm, 99.5 to 100 works just fine. I try not to let it stay any lower than 99.5. I have had better luck slightly warm than slightly cold
I read it on byc, but can't remember who posted the thread. Anyways I'm trying it now. My temps in the hatcher have been ranging between 98.8/99.3 - my weasel has been about 99. So far I've had 8 hatch and I still have 8 pipped and only few eggs that haven't. Day 21 just started yesterday for 1/2 of them and the rest of the eggs day 21 starts at 2 p.m. today. The first 18 days they were in my Genesis which held at 99.5 the whole time.
I have a 1502 too as well. The hatching tray is at the very bottom of the cabinet. The thermometer is at the top. I imagine this is by design and the bottom is 1-degree cooler (or so). So, I don't have to bother with the thermostat.