I suspect you did not understand what MarkT explained in post #7 here: how to control nichrome wire with arduino? - #10 by Carsten4P - Motors, Mechanics, Power and CNC - Arduino Forum
The better approach is to monitor the power applied to estimate temperature rise and resistance. Trying to measure resistance is extermely difficult when dealing with a milliohm device with a relatively low temperature coefficient of resistance.
Depending upon your operating voltage and current, you can use something like the INA219 breakout boards to calculate the power directly.