A 250 W Halogen bulb will give of a certain no of lumens. 1715 lumen = 1 KW/m^2.
The Al relector will reflect part of the energy and absorb some to heat itself up. The reflection / absorbtion ratio will depend on many factors such as the surface treatment of the Al reflector.
HOw hot the reflector will get will then be determined by the thickness od the reflector and it's insulation as a lot of the heat will be immediately lost to the ambient air.
Too many variables. m777182 is right make a test set up and measure it. Change ambient temperatures. Air speed and reflector position to try to establish correlation.
best regards.
scalleke