Yeah what slim6y said, what seed are you trying to germinate? The altitude and climate of the origin of the particular sp. will give you an idea of what you need. From exprience almost any cacti will germ at 25'C and 80% humidity. But once you start looking at sp. such as Ariocarpus you nee to be quite precise for good germ.
A note on the variance, day/night issue. I don't believe a variance is essential to the germination of cacti but from exprience a better germination is achieved when the temp and humidity drop moderately over night. In addition light must also be cycled with a 6-10 hour down period.