Serpentine PCB heater size calculation
The calculator temperature rise figures are not going to be accurate- you'd be better off to use another method to estimate the rise of an isothermal plate the size of your PCB from the total wattage. 

Copper has a positive temperature coefficient of about +0.39%/K so you'll have about 25% less power at 90°C, however you also should never exceed the maximum current output of your power supply, so  you probably should aim for a bit higher than 4 ohms. The copper thickness is also not all that well controlled, so unless you can adjust the voltage a bit you might want to be a bit cautious. You could add some taps to trim the cold resistance. 

I get more like 3\$\Omega\$ @ 25°C from first principles using \$\rho\$ = 1.72E-8 \$\Omega-\text m\$ for the resistivity of copper  for 7m of 1.14mm wide 34.8um thick trace. That's probably worth looking into. 

I didn't look at the other calculations- there will be some effect from the shape of the corners (sharp vs. radiused).