Understanding Photo Transistor Method Results

Photo Transistor results are uploaded to a separate sheet. These are captured off a real monitor (Inzone M3 generally, given that it is as close to a standard FGC monitor as we are going to get.) The setup I use is to have a pair of Phototransistors, one towards the top of the screen and one towards the bottom. This gives a pair of results, which are the Read1 in blue and Read2 in green. Using how far these are separated the top and bottom (red) results can be extrapolated. 

For example, if Read1 gave 59ms and Read2 gave 61ms, and they were at 25% and 75% of the way down the screen, we could assume that it takes 2ms to draw 50% of the screen, or 4ms to draw 100% of the screen. Therefore the top value should be 1ms (25%) less than the Read1 value and the bottom value should be 25% (1ms) more than the bottom value, hence 62ms. It also backs up the idea that if the screen takes ~4ms to draw, then the screen refresh rate should be 240hz.