Hey Kuzar, it doesn't need to be precisely 90F all the time. It's natural for everything, including the hot spot, to be a little cooler at night. Just make very sure it's never getting above 95F at the hottest time of the day, and check it again in another day. Once you get it adjusted properly, you can draw a mark on the dimmer with white-out or a washable marker, and then you shouldn't have to mess with it more than once a month.
Yeah, a real helix or herpstat would be better. But if this is your first and only animal, you should be able to make this work for your situation for a good long while.