Don't know if there's any accurate universal formula for adjusting your pace count.

One simple way to check the effect for particular gradients is to make two marks (with a known distance between). Start from one mark and walk uphill to the other, counting paces, then rotate 180 and starting from zero, count paces back to the point. Try a few times for average, and compare both counts to your average on similar but level terrain.

EDIT: And dead reckoning means determining your position from your previous known position + average speed and duration to direction travelled.

Dead reckoning is pretty accurate when on a sailboat in fog, trying to include currents and wind to your calculations. Not fun.

-jh