This is the buildout of the heated bed used on the Arcus 3D C1.
For this project, I was aiming for a 400w bed at 120v. That is a little aggressive for the size, but I don't like having to deal with part cooling dropping my bed temperature either.
A little Ohm's law says 400W at 120v(RMS) works out to 36 Ohms required resistance. But... the resistance per foot of nichrome wire actually increases in resistance a bit as you heat it. So I aimed for 33 Ohms instead.
I have 24 gauge nichrome wire. So per the above chart I would need 20.6ft of it to hit the target 33 Ohms.
My granite tile was 12x12in. Laying that out on the tile, leaving 3/4in from all edges resulted in a roughly 21ft path if my cuts are placed 1/2in apart. I like nice round numbers.
Made the cuts, ran the wire, terminated the ends and encased it all in furnace cement. Instructions below.
After construction my measured resistance is 33.6Ohms. Not bad at all for a hand wound ceramic 400w resistor. :)