Calculate pressure of air trapped underwater

The pressure in the straw is (almost) the same throughout the straw. I say almost because air does have some weight so the pressure would be slightly higher at the bottom of the straw than the top, but this is going to be a very small effect.

The pressure at the air/water interface has to be the same as the pressure of the water, otherwise the water would flow into or out of the straw until the pressures equalised. So if you can calculate the pressure of the water you can calculate the pressure of the air in the straw.

The flex of the walls of the straw doesn't affect this concusion. However it does affect the volume of the straw so if you're using the distance the water enters the straw you will have to correct for a volume increase. If the straw is stretchy the water will be able to enter the straw further for the same pressure.

Temperature will change the density of the water and therefore the pressure. You need a table of water density:temp, or some suitable approximate equation to estimate it. I don't think humidity will have much effect, though I'm not sure I'd swear to this in court.