Ah, yeah, I should have asked where the temperature sensor was, as there’s always a temperature gradient between the heating element and the surface of an iron. In theory, you could calibrate it to display the surface temperature, but it wouldn’t really be worth your time – once you find the correct temperature by trial and error, you can just “set it and forget it.”
I recently added a video to the Proper Heat-Sealing Technique page under the How-To menu to show how the fabric behaves when the iron is set to the right temperature.
I’m obviously an advocate of the DIY approach, but after modifying several different irons myself with mixed success, I’ve come to the conclusion that for most people it’s worth spending $20-30 on a reliable iron that works well straight out of the box instead of spending a bunch of time making one from scratch or modifying an iron that was designed for a different purpose. If you’ve got the skills and the inclination to tinker with electronics though, that’s a totally different story!