Hi happy - since you have a mixed bag of seeds go for 75 - it's a bit low for some and a bit high for others but should work. If you see the soil drying out too quickly then lower it to 70. Keep in mind that with a waterbed mat you have to have the temp probe stuck into a cup of soil too. I just fill a separate coffee cup with my damp potting mix and stick the temp probe in it and set it on the heat mat. It controls the heat cycle of the mat.

One other tip - sometimes setting the trays directly on the heat mat can melt and warp your tray. Small 1/4" spacers of wood under the corners help prevent that while still letting the soil heat.

Good luck! ;)

Dave