Measuring the real distance between two points from a non-perpendicular taken image

I am doing a laboratory tests about the spread of the fire, I have a rectangular fuel bed and I want to calculate the rate of the fire spread which requires to know how much distance the fire front have passed from a frame to another, so simply I need the get position (x,y) of any point on that rectangle (the fuel bed), The problem is that the camera is not on the perpendicular line to that rectangle, so I need to calibrate the camera first to align the image so I can get the real positions, I have seen the Single Camera Calibration app on the Computer Vision Tool Box but it requires a checkbox which is impossible to put it on the same surface (the fuel bed), it will burn! So I need to know if there is any other way to do it and get (x,y) positions or measure a distance on that surface I hope I explained well my issue here