Make sure that both left and right side are jacked up and both wheels are taken off. With one wheel on the ground there can be some preload on the opposite side and measurement will not be consistent.
I use measuring tape - make sure that spring lenght is the same and total length of the shock is the same.
It will always look and measure bit different on the ground. Unless you can find perfectly level slab of concrete car will appear as bit uneven.
I just installed M7 kit, no instructions provided. I got it even right away, issue was to get to desired right height as turning collars 5 mm could give very unexpected results-from nothing to ugly
For the rear-I set up ride height without shocks installed-actually put wheels on and put the car on the ground to see if OK. After that I adjusted shock body length to be about 5mm longer than shock mount bushing . No idea if this is the correct way, but rides great.