How do you determine "ride height" to make suspension mods??

Well I am about to get my triangulated 4 link set up in the works and everyone keeps telling me I need to FIRST determine my " Ride Height " in order to get the car set where it needs to be so all of the brackets can be welded in at the proper places.

Well how do you determine this?? Is there a standard?? Or formulae??

Here is what I am working / my game plan:
I have a 71 Demon with a mini tub and 3" spring frame mount relocating kit already installed.

My game plan for rims and tires are as follows:
Rear Rims: 18" x 10" - Bullitt Wheels
Front Rims: 18" x 8" or 17" x 8" - Bullitt Wheels

Rear Tires: No clue what will fit yet, but would want to get a big tire in rear. Something like a 335 or 345 tire leaning towards a BF Goodrich KDW.

Front Tires: No clue what will fit yet, but would want to get a big tire in rear. Something like a 245 tire leaning towards a BF Goodrich KDW.

Tire issue is I have always been under the impression that I can't get my rims and tires till I get my rear end 4 link done and actually get everything measured up??

As for my rearend: I currently have a stock A- body 8.75 with lg bolt moser axels, housing is back braced, and sping perches have been moved in for the 3" kit.

Can anyone help me determine how to get my ride height determined so that i can get my fabricator to set the car at the right height when we starts my triangulated 4 link. Or at least point me in the right direction so I know where to head from here.

Thanks!