grandpabear, You're in the ball park .... as TheMrNotSoFamous stated, if the three loads fall in the linear portion of the pressure curve, a simple math function will get you pretty close. In this case, the high end load has entered the non-linear portion of the pressure curve so it's pressure is a bit high. That means the 9.8gr load will be about 19k psi .... a couple k psi lower than expected.

The problem with trying to compute pressure mathematically, you have no way of knowing if all three points are within the linear portion of the pressure curve. Linear burn rates are affected by bullet weight, internal case capacity, and of course powder charge weight. As an example, at lower chamber pressures, a tenth of a grain increase in powder charge will result in a predictable change in chamber pressure. As you go up in charge weight a couple grains, you will find that same tenth of a grain increase will result in a much higher change in chamber pressure. This is an obvious indication the charge weight is getting out of the linear range. It's very rare to find a powder with more than a 2 grain max linear range, let alone 4 grains in your example. This is where QuickLOAD comes in handy.

Edited to add: I guess I should qualify my statements before our engineering friends beat me up. It's virtually impossible for a powder to be truly linear ... meaning each .1 gr increase in charge weight will increase pressure by x psi. The reason being ... as powder is added to a case, it also adds to the projectile weight. As an example, let's say you had a 150gr bullet with a 5 grain powder charge. The total weight pushed by chamber pressure would be 155 grains. If you added 1 grain, now the total weight would be 156 grains. Further, lets say a powder charge uses 60 % of the internal case volume. If you add a grain of powder, it would reduce the free space in the case where the powder now occupies 70%. Both factors (more powder weight and less free space) increase chamber pressure a little. So .... the pressure trace looks almost straight but in fact is slightly non-linear .... similar to Varminterror'a above analogy with the curvature of the earth.

Every powder has its unique properties that change its burn rate ... bullet weight, bullet diameter, and bullet seating depth. If you look in any reloading manual, you will see fast burning powders have a very narrow linear range ... usually well under 1 grain from the lightest to the heaviest load, mid-burn rate powders have a wider linear range and slow burning powders can have an even wider linear range .... maybe as much as 3 grains. That said, just because loads are listed from minimum to maximum charge weights, it doesn't mean the inclusive loads are in the most linear part of the pressure curve.