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

It really depends on intended use and what you want your final look to be. I used a very interesting method to determine my ride height. I was looking to go as low as possible so I set the rear frame on the axle housing then raised the car 3" which gives me 2" of travel and a 1" bump stop. I made my own triangulated 4 link and I just built the bars from there.

On the 18x8's in the front Plan on running a 225/45/18 tires and at least 6" backspace if you want to be able to turn the wheel. I am going to stretch my fender openings so I can run a bigger tire in the front.