A heating pad isn't enough to be the sole heating source. You need a thermometer so you know for sure what temperature her cage is at. It's too hard to tell the exact temperature just by feel, and hedgies can be sensitive to the temperature being off by only a couple degrees. If you want to keep the heating pad under her sleep area, that's fine, but you need to be sure that the air temperature is staying at a steady temperature that's safe. It's not great to depend on a heating vent, since even drafts of warm air aren't good, and if the heat is turning on/off, the temperature may be going up and down too much and trigger an attempt. There's a couple other options that you can use to keep the air at a steady, safe temperature, like a CHE or a space heater. Heating options are explained more in this thread - viewtopic.php?f=8&t=4579
I wouldn't turn the heating pad up too much. It may not feel too hot to you, with your hand on it, but for a hedgehog laying on it for long periods of time, they can still get burned just from prolonged exposure to the heat, if it's even a little too hot.