I’m not sure why you have that piston for a magnum head but it’s not correct. I’m not into running thicker head gaskets just to fix that.
It’s difficult to measure just the positive deck height unless you do a 1/2 inch downfill. I’ll give you an example of what you do.
You put the piston with rings and rods attached and the crank in the block and install it in the number 1 hole. Get a dial indicator and find TDC. Then measure .500 down the hole. I usually go .600 down and then put some grease around the rings to seal them and then raise the piston back up to .500 on the indicator.
Take your graduated burette and fill the cylinder to the top. You need a clear acrylic flat piece with a small hole in it to fill it up. You can use denatured alcohol, solvent and I know some that use water with a little dish soap in it.
Once the cylinder is full you see how much it held. So as an example I’ll use a .030 360 for the math. You simply find the volume of the cylinder by the equation of bore x bore x stoke x 12.87 and that gives the volume of the cylinder with a flat piston and no valve reliefs or anything. In this case the answer is 836.608 or 836 cc’s. If your quench pax is actually 25 cc’s, then your actual down fill would be 811 cc’s. So you know you have a 25 cc quench pad. You also need to know how far out of the deck or below the deck the piston is. For a simple example will call it a zero deck
Now all you need is combustion chamber volume in cc’s and the head gasket diameter and thickness and you can calculate you actually compression ratio.
So let’s say you have 65 cc heads. You head gasket is .040 thick with a 4.090 bore.
The math is the same as above. It’s bore x bore x thickness x 12.87 or 8.6116 or 8.6 cc’s
You now have the volume of your cylinder at BDC and you know what the actual combustion chamber volume is because it is chamber volume + gasket volume + any deck volume below deck or you subtract the number of the quench dome form your chamber volume.
So...let’s do the math. We know the chamber is 65 cc’s and the gasket is 8.6 so that’s 73.6 cc’s MINUS the 25 cc’s for your quench dome so you have a net chamber volume of 48.6 cc’s.
Now it is your cylinder volume at BDC divided by the cylinder volume at TDC and you’ll get your actual compression ratio.
So in this example you have 811 cc’s at BDC.
Thats 811 divided by 48.6 which is 16.687 CR.
Obviously this is a high CR but I doubt the quench dome is 25 cc’s and I doubt the piston is zero deck but probably down the hole a bit.
That’s why you have to measure this stuff. The hard part is measuring it. The math is easy. I know it’s easy because I can do it and I hate math.
Edit: I just saw you have a dish in the piston so your down fill would be a bigger number because of the dish, so you’ll see that when you measure the down fill as the dish offsets the volume of the quench dome. And the only way to know by how much is to down fill it.
