Recommended air pressure I have seen is 50 lbs for flexible underground irrigation line, 80 lbs for rigid PVC lines.
It takes 60-80 lbs for my spring-loaded heads to fully pop up, so I use 80 lbs.
I have seen videos that say the air should be introduced after the back-flow preventer valve.
This requires an additional fitting between the shut off valve downstream from the back-flow preventer and the valve box.
Other videos connect the air to the pressure relief valve and run air through the back-flow preventer.
I had my sprinkler system blown out by different professionals the first few years I lived in the current house and they all ran the air through the back-flow preventer valve, so that is what I do.
The reason I started doing my own sprinkler blow outs was scheduling problems with the professionals.
If a hard freeze was predicted like this week, they usually were booked and advised me to drain the system and wrap the back flow preventer until they could get to me.