Here's something else to consider - not everyone uses heat mats. I have used them on seedlings and not, and have had more issues when using them.
As long as your living environment is what humans usually prefer, and the seeds are in that environment, it should already be in the low 70's.
Your T5 over a closed dome is going to warm it up into the comfort zone for seeds.
Some people leave the light on 24/0 for warmth until seeds pop, then switch to 18/6 or so.
I only use the heat mat for soaking the seeds now. and I use 18/6 light for warmth as soon as I plant them. Keeps me from forgetting to shift the lights back, and they should only cool down to 72F at my house once they are dark. .