Electrical – Zero cross circuit exploded

The calculated angle is accidental, it is not a reliable value. To demonstrate how unreliable is it, imagine that capacitor is 1 pF. The angle will be close to zero, << much less << than 270.

I'd suggest to add optocoupler with high voltage diode and resistor, read zero crossing with accurate estimate of current transfer ratio of optocoupler, known part's delays, and known software delays. Though with high voltages (above 300V) the coupler should be super low current kind, say with LED requiring less than 1 mA to be detectable.

A calibration value can be automatically created if you consider using a spare analogue input on the PIC. Connect this input to a potted-down and biased version of the AC and let the PIC process mean and peak levels in order to determine a reference zero cross point.

Then let your firmware make an estimate of the delay and use this as a calibration value for the opto-coupler. This would be a one-ff test and probably it would be sensible to use an AC isolating transformer because the resistive divider doesn't isolate.

Alternatively, you could "rewire" the opto temporarily and "hit" it with a PIC digital output onto the LED. Then your firmware can measure the time delay.