High g's come mostly from hard pulls and not from tumbles. Here is the equation for centripetal acceleration, (v^2)/r (velocity squared divided by radius of path of travel). This is why a redbull air race pilot going 200 mph and then pulling into a TIGHT half cuban turn-around will pull 12 g's, but a blue angle going 350 mph will only pull 4 or so while flying a HUGE loop.
Pretty sure that's the only equation I remember from dynamics...
To figure the g's in a parachute or wall maneuver, I guess you would need to be able to measure the speed of the plane so you could calculate momentum. Or just install a g meter.