In any gun, muzzle energy is equal to the average (not peak) pressure behind the projectile all the way down the tube, multiplied by the length of the projectile travel down the tube. Thus, the average pressure is proportional to the square root of the velocity. But the proportion of the peak pressure to the lower pressure at the muzzle or other places in the barrel that go into making up that average is not knowable from velocity. So there is no way to measure peak pressure directly from velocity.
What you can do, and I expect this is what they meant, is look up loads with the same powder and primer and ejecta (pellets, wads, powder gases) mass and a provided powder charge and see how your velocity compares to theirs, assuming you have the same barrel length they used to collect their test data.
Measuring the velocity over an optical chronograph is risky, as even if you do it before the shot pattern spreads out, the wadding can still strike and damage the machine. Usually people put some kind of mask up to protect the chronograph, like a pine board with a hole through it that lets the shot through. The the first pellet should trip the machine and stop it, giving you maximum pellet velocity. The trailing pellets will be going slower, but they will be going.
I don't know how the Labradar and Magnetospeed would handle this. The Labradar might tend to track the wad's velocity and even if not, would tend to follow the trailing pellets and so give you a reading corresponding to the lowest velocity in the shot string. The Magnetospeed I think would favor the leading edge. You could call the maker and ask.
This is a funny situation in which an old fashioned ballistic pendulum might actually give you the best average velocity reading, though it is hard to get a lot of resolution out of one.
The alternative is to invest in a Pressure Trace instrument and get actual pressure readings you can compare to commercial shells fired in the same gun. It costs as much as many guns, so this would have to be something you want anyway for checking out other guns you own.
