Well the angle is of no relitive importance of how effective it will work.

That being said is that the angle determines where the heat will be reflected.

Say you are camping a flat 90 degree reflector place beyond the fire will direct it towards the shelter. Depending upon size angling it slightly towards the fire will direct it somewhat downward keeping it from passing over the shelter.

As to the shelter itelf most are slanted at about 45 degrees. This is to direct the heat it reflects down toward where your body will be while sleeping.

Basically play with the angles to im the heat whhere you want it to go.