I've heard 150mm (+ your head tube) used as a general 'rule of thumb' to account for stack height of the headset + spacers, but that may be too high (as its possible to cut down a fork that is too long vs. having one that is too short thus unusable).

Do you know the stack height of your headset? Or the height of your stem? Depending on the material & manufacturer of the fork, there should be a spec for what is acceptable for the total height of spacers used. Add these up & you should get a useable # for selecting your fork.