There is a formula. To solve this problem, two steps are necessary:
1. Finding air density (rho)
rho = p/(R x T), where
p= air pressure in mb
R=Gas-constant (=287,04 m^2 / s^2 x K)
T= Temperature in Kelvin (=°C + 273)
2. Applying the formula:
dh= -1 / ( g x rho), where
dh=interval corresponding to 1mb press.change expressed in METERS
g=acceleration (=9,80665 m/s^2)
rho=air density as solved above
If you would be very picky, you could also account for decreasing acceleration at 25000ft! Nevertheless this should yield quite accurate results.