Nope can't be done. The temperature doesn't depend on the power of the heating mat. It depends on the total energy supplied, less the energy lost. Low power, plus good insulation will get very hot if you wait long enough. High power and no insulation means the surface of the soil will always be at about the air temperature of the room.

Experiment. Use insulation to get the whole tray uniformly warm. 50W should be enough for bottom heat on half a dozen trays. Lastly, get a thermometer with a probe to take the soil temperature. Your hand can't tell the difference between 70F and 90F, it interprets wet as cold, plus the same temperature feels cold when the air is warm and warm when the air is cold.. Skin just isn't a good thermometer.

Thanks, Shrubs.

What I was shooting for was converting joules per second to degrees temperature. I have a conversion to heat calories, that's sort of ...

I was just thinking that if I knew the values for the heat input I could get an idea of the heat of the soil where the seedlings are.

See, that heat has to rise to the surface. Put heat in at the bottom, get heat out at the top.

After the specific heat capacity of the potting soil is all warmed up, the heat will rise to the surface and then convect and radiate away.

But it's the warmth of the top 1/4 inch of soil I wanted to know.

But just like with everything else in the world, you have to measure it to know.