From what I can tell from the manufactures of tent heaters their focus is on providing BTUH temperature rise a calculation made based on square feet and estimated temperature desired. 

Example:  Tent size 40'x80'=3200SQFT say it is 40 degree outside and the desired temperature is 70 degree requiring a 30 degree rise in temperature.  This would require a 24,000 BTUH tent heater.  However, if you take the average person at rest per hour of 356 BTUH and multiply 320 people sitting under a 40x80 tent (356 BTUH per person x320 guests = 113,920BTUH).  This does not mean you don't need a tent heater as the thermal insulative property of 16 ounce vinyl and the amount of thermal loss from wall seams & doorways etc. equates to substantial thermal loss.  The tent heater is intended to supplement the environmental temperature of the tent by improving rise temperature.  Sam's Rental is evaluating thermostats for our tent heaters to improve your ability to control the temperature environment in the tent.