You can only use this equation if the two waves are the same period (which you can verify by calculating the period of both of them and comparing). Phase difference between two waves of different periods/frequencies conceptually makes no sense, as the difference would be changing with time.
Calculating the minus of the two periods will only find the difference between them, which will be 0 for any situation where a phase difference conceptually makes any sense.
Choose two identical points on one of the waves, and measure the time between them. The easiest point is probably to use the positive-going 0 crossing point of the blue wave, as its conveniently close to a grid line for accuracy.
This is because the period is defined as the time it takes for one cycle to occur.
It looks like there is 5 'boxes' (called a 'Div' or 'division') between the first and second positive going crossing, and since the timebase is 200us per Div, the time period is 5 x 200us = 1ms.
So by my measurements, phase difference will be (220us/1000us)*360 = 79.2 degrees