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

Lots of good advise here, a couple of thoughts:

Bump steer the car (measure the suspension travel front and rear, use the current ride height measurements to establish the base line and make your decisions from there. You need to understand the base line and travel to understand where to relocate the rear suspension and what to set it at. There are other considerations like what the car is used for and weight percentages for handling characteristics that drive set up. Chassis alignment as it relates to the front end geometry is also a consideration again determined by what you want to do with the car. Widths are pretty simple to measure, your toe plate idea would work well for that purpose, once you have that you can then make some tire and wheel size calculations. You can impact the ride and stance with wheel sizing also so so that should be part of the total calculation depending again on what you are looking for. Be certain you understand how to measure backspacing on wheels (much written on the subject). Then be certain you understand the carcass construction and tire size variation manufacture to manufacture (they publish the info) it varies. Also understand how the wheel relates to the tire if you stretch the sidewall or make it as tall as possible. When the MFG lists tread, cross section and height they will usually say on what size wheels. Understand how the total tire travel impacts things like fenders at the widest section of the tire. Tires and wheels are usually last for me, hope that helps.