Tech Question - Mustang Ride Height

Hello Mike,
The car I'm shopping rear springs for is a 1968 Ford Mustang Fastback.
It'll be equipped with a 428 and nine inch rear with a 4-speed.
I have a particular size tire I plan on running on the rear.
The size I'm looking at is 28x10.5x15.
The springs you list come as standard or +1, +2, or +3 and of course some minus's.
My question is what or how do you reference your plus and minus lifts and drops from?
I've been researching this and the buzz words tossed around are "ride height".
Those measurements don't help me.
With all the different size tires people run on their cars today combined with old sagging springs and various vendor's interpretations of plus and minus lifts/drops there's nothing definitive.
What would really help me would be to know the factory/original distance from the horizontal centerline of the rear axle to the rear fender lip or some other reference point on the car.
I don't know if that is how you gauge whether a spring will be plus 1", 2", or 3" additional lift over factory.
I can find info on the "factory ride height" all day, but that was with 14" wheels along with all the other variables.

Thanks in advance,
Bill

Bill,
Sometimes it takes a little reverse engineering to come up with he answer you need.
Just as out chart says, our ride heights are based off of the original stock ride heights.
And as you state the ride heights found are based upon the stock 6.95 x 14 tires.
When other tires are used, the figures must be adjusted for the variation, if any, of the tire diameter.
A 6.95 x 14 tire has a diameter of 25.3 Inches. I am guessing the tire you are using has a 28 Inch diameter.
If so then 1/2 of the difference between 28 and 25.3, 1.35 Inches must be added to the ride height chart.
When we calculate ride heights, we use whatever measuements the factory originally used.
Ride heights for the '68 are measured from the ground to the bottom of the wheel well.
And for the '68 with factory size tires range from 25.125 to 26.875 for the standard suspension cars and from 24.75 to 26.5 for the Improved Handling and GT suspension cars.
Actual dimensions are based upon the body style of the car.
The best thing to do is put the tires you will be running on the car. Measure where she sits and compare it to the "adjusted" stock ride height.
Then jack her up to where you would like her to be sitting and re-measure.
It is the difference between the 2 measurements that will tell you how much over stock you need.

Mike