John is it the pressure altitude itself you are looking for?
One doesn't really need a formula as such for it if one already knows the field elevation and local altimeter setting.....
to take your example:
Field elevation 2000ft
Altimeter 3092
So the altimeter reads 2000ft with 3092 set. If you then set 2992 or if you don't have the altimeter to hand just take off 1000ft for every inch of mercury above standard atmosphere one happens to be you then get the pressure altitude. So in your case it would be 1000ft.