Well, 98.8 to 100.4 averages to 99 inside of the egg. You're incubator is way better than most of the stryo foam incubators on the market. I think that what you have is good.
If you are trying for better temperatures I would go ahead and put the light fixture just a little closer to the thermostat.
I don't use a water wiggler, but when you hear about how a lot of peoples incubators. Stay right at 99.5 degrees, it is usually because they are using a water wiggler. The air temperature is bound to fluctuate, but an egg has mass on its side. An eggs mass retains lost heat in the air, or if it has gotten too high, it doesn't usually make a difference.
(I didn't say that there was anything wrong with using a water wiggler, but that a water wiggler shows the average temperature.)
I recommend buying a water wiggler, and seeing what your temperature is then.
If you are trying for better temperatures I would go ahead and put the light fixture just a little closer to the thermostat.
I don't use a water wiggler, but when you hear about how a lot of peoples incubators. Stay right at 99.5 degrees, it is usually because they are using a water wiggler. The air temperature is bound to fluctuate, but an egg has mass on its side. An eggs mass retains lost heat in the air, or if it has gotten too high, it doesn't usually make a difference.
(I didn't say that there was anything wrong with using a water wiggler, but that a water wiggler shows the average temperature.)
I recommend buying a water wiggler, and seeing what your temperature is then.