What kind of light? If it's a LED you don't need to keep the tent open just the filtered vents. It'll be the same temp inside and out give or take 2 degrees.
I hook my fan to a thermostat. So it stays at a set temperature. If the fan can keep up to the light it's a good system. Otherwise you have to open the tent up.
Have it sucking out of the tent from the top... open the flaps on the bottom to draw in cool air.
Put your temp probe at canopy height out of the light. Can't count how many times I seen sensors I the wrong g place and ppl think thier temps are an issue.
Tbh you want to go by leaf temp not air temp as that's what's important.