It's not going to be staying at a perfect 99.5 the whole time through no matter what you do. This last winter I ran mine at 100.9 because I couldn't get it above 98 otherwise. Not sure why. This time I have it at 100.2, close enough. I think it's the digital thermometers and their accuracy...
I use the Hovabator 1602N as well. Excellent results with local eggs, mediocre with shipped eggs, but I usually get something. The first time I ever used it, it took me 3 days to get it going correctly. I hear the LG is more temperamental, even though they are the same style.
It can take HOURS...