temperature gradient is the difference between the temperature of the object to be cooled and the heatsink?
surface area is how big is the surface area I will be cooling?
thickness for conduction is thickness of the thermal pad?
So I could get away with a cheap thermal pad on the low power part? I see...
Yep on everything. You can also easily test this with a cheap pad and a "good" 12W/mK pad. You likely won't see any difference on the stable temperature of the thing. If you're choosing pad thickness: as thin as you can go while still getting full contact (thermal paste is better than pads because the paste thins out to almost-nothing)
For stuff like watercooling it's completely linear and why you'll hear "delta T" so much. "a 240 rad can handle 300W at delta T [water temperature vs. air temperature] of 20 degrees" or something like this...which tells you it'll handle 450W if your system can tolerate a ambient + 30C water temperature.
(this is all like physics 101 level....the 401 level is crazy shit about Fourier equations and by that point in my college career I had no fucks left to give so I don't remember it: https://en.wikipedia.org/wiki/Heat_equation ... nowadays everyone just plugs this shit into CAD software to do the modeling and math for them)