Check the upper and lower mounts for bends or damage first, then proceed.
Unless the car has the stock suspension and new, factory spec springs, your rear ride height numbers are really meaningless.
Just like front ride height numbers. The factory only did it one way. Stock/stock/stock.
Otherwise you need to disconnect your shocks and raise the body, letting the rear axle hang loose.
When the tires clear the ground, measure the distance between the mounting studs. That is your extended length.
Now lower the car and put weight in the trunk until the rear axle bottoms on the upper frame rubbers. Re-measure. This is your collapsed length.
The difference is the actual stroke of the shock absorber.
These are the numbers you will use to shop for the proper fitting shock for your particular car.
The shock companies have a chart which has the extended and collapsed measurements on their shocks as well as the mounting types.
But no car with a 45 series tire will ever have a "good ride" .
You tires and rims will just love potholes. Bent rims and pinched sidewalls are in your future...