If you put a digital thermometer's probe under the light and leave it, you'll be able to monitor the temps. If too cool go for a higher wattage lamp and if too hot go lower. I think either a 25-40w bulb will do you, as you don't need that much extra heat. There not as far as I'm aware but when you use heat lamps for a while and in many vivs you'll get an idea of what you'll roughly need