E71 TPMS slow warning or normal?

[conandrum]

Hi all,

I have a BMW X6 E71 2008.
My tires were checked 8 weeks ago - (when I changed my rotors/pads and painted the calipers).

Yesterday I climbed a pavement (no ramp) and parked the car.
Came back 5 hours later and climbed down slowly and drove 3 miles before the TPMS warning came on regarding my rear right tire.

I checked both rears - left:2.54 right:1.8x
Filled the right one to 2.6 and has not lost anything yet.

Questions:
1. Why did the system wait till 1.8x bar to alert me (does it alert when it drops 30%)?
2. Can it be programmed to alert me sooner? Can I do it myself with some software?
3. If I wanted an active TPMS, is my only option a TPMS with 'external sensors' since I probably should not touch the internal ones?