Sometimes, just using shear pins is not enough.
If you trust all of its calculations sufficiently, you can use OpenRocket's component analysis to compute what percent of the drag on the rocket comes from forward of the break. Then, figure out what percentage of the mass is forward of the break.
The drag separation force (disregarding internal pressure) will be:
(deceleration of rocket(t)*mass of front) - (% of drag from front(velocity(t))*total drag force(t))
For a given deceleration of the rocket, the total force needed to keep the front part attached will come from two sources: the friction at the break and the drag on the front. Subtracting the drag at the front will yield the amount of friction at the break.
Note that for supersonic rockets, the distribution of drag force will vary greatly with Mach number, so be sure to record a few values: the highest Mach numbers will be when the magnitude of the drag force is highest, but it might not be the point when the drag separation force peaks.
I wish you could set a separation point in OpenRocket, so that it could output drag from forward and aft of the separation point in graphs. Then I could set up a formula to plot drag separation force versus time.
Temporarily potential impulse transformed into no-longer-potential-anymore impulse since 2013-01-01: A lot.