That would get you really close, BUT, it would be very hard to compress the shock due to the N2 charge and the bump zone.another way would be to take the shock off... Measure the collapsed shock height (no shaft showing) with the rubber bumper taken out of the equation.. Get that measurement.. now lower the car (no shock) and measure from upper and lower mounts to the same length as the compressed shock. This should get you really close.
Of course letting the nitrogen out would help.
Another easy way is to measure eye to eye, then subtract the amount of shaft showing, and maybe half a bumper.
You could possibly do this on the car, with the springs, by disconnecting the lower shock mount and swing the shock out of the way.
It would be very accurate to just put the shocks back on, minus springs, lower the car, and compress them fully.
The weight of the car should compress the bumpers a little, but a hard bottom out could compress them 50%, more or less.
Just don't lower the car to the floor and then try to figure out how you will get it off the floor.
If you put blocks under your tires, that would allow room for your jack.