Up to how high of sustained winds/wind gusts can one operate the phantom safely?

Yesterday it was rather windy with gusts probably up to 20Mi/hr where there were some challenges but still could manage OK.   Today winds are much higher, sustained winds are averaging 20Mi/hr with gusts probably 25-30 so I decided not to fly.   I was hoping I could show off my P3A to friends in flight today but didn't want to risk it.

Probably a good decision.

That depends on what kind of flying you want to do.
The Phantom can handle more wind than most users imagine if handles properly.
Your Phantom can make about 35 mph in still air and could hold position in 30 mph winds.
But you may have difficulties launching and landing in those conditions.
You couldn't make much headway against a wind that strong and if you went downwind you would have trouble coming back.
Plus winds aloft are always stronger than what you experience on the ground.
If you are up 500 feet or more they can be a lot stronger.

In winds of up to 15 knots at moderate distances and altitudes your Phantom should have no problem.

I figured 30mph would be the practical upper limit.

I assume a good method to determine winds at altitude is to switch to ATTI mode at the desired altitudes and observe the horizontal speed and direction of drift before switching back to P-GPS mode. This would provide a comfortable ceiling for the current flight.

I flew a datamapper mission in 25mph sustained with 35mph gusts last week and it handled surprisingly well, only a few photos had a bit of the props in the frames during the turns, which I cropped out before running the data. Just plan your battery appropriately. We flew a P3A and P4 that day, both did well.