Circular Polarized Antenna Gain Measuement

I built a CP antenna and would like to find the gain of it. I have a 2 linearly polarized horn antennas, which have known gain.
My technique in an anechoic chamber:
The horn is placed 35 inches above the CP antenna; the CP antenna is spun 360 degrees; the power received is captured for every two degrees as Horn_CP

The CP antenna is replaced by the second horn and the power received is captured as Horn_Horn
The axial ratio is calculated as Max(Power Received) - Min(Power Received) for the Horn_CP measurement
The gain correction factor is calculated given in book and based on the axial ratio

Gain_CP = Horn_CP - Horn_Horn + Gain_Horn + Gain_Correction

I can't seem to get the correct gain based on what I simulated. I simulated -3dB at 1575MHz and got 1.5dB based on my measurements . Am I using the correct method of calculation Gain for the CP antenna or is there another method I should use?

Thanks in advance