Adding spacer at the top and lowering the spring perch at the bottom to offset it has zero effect on the characteristics of the suspension, and the preload of the spring doesn't change (if it did, as the damper body length doesn't change, the ride height of the car would change!)
The body length/preload adjustment is a common misconception that has started with JDM type coilovers with body length and spring height adjustment (I used to also only adjust suspension with the body length 10+ years ago so I know where you're coming from!). You'll notice that on any real top tier racecars that there isn't anybody length adjustment - just spring perch offset. Why?
The reason is that the body length is set for the parameters of the car. You know the travel range you want (i.e. the car should not be able to hit the ground, no suspension arms should hit the chassis), and so the damper length is set so that at full compression, with the bump rubber nearly fully compressed, the body of the car is close to but not hitting the ground after taking into consideration tire deflection.
Once that is set, there is no reason to ever adjust it unless you're changing something major (like tire diameter or drop spindles for example). At that point, all height adjustment is done with the spring perch offset.
How much "preload" is on the spring at full droop is really an inconsequential value. In fact, if the damper has enough travel (as our AWD sports coilovers do) you can have so much droop that you have almost no preload when the suspension is fully extended. This gives the best ride quality over large pot-holes and drops in the road, as there is little to no chance that the suspension will ever max out on droop.
If you've ridden in cars that have only 1" of droop or so, you'll know that they ride horribly, even with soft springs. Why? Because as soon as a wheel encounters a dip in the road greater than the amount of droop available, the suspension effectively goes solid and the car falls into the hole!
For future suspension installs with those types of coilovers my advice is to set the body length so that the damper has 40% droop travel and 60% compression travel, assuming the car won't hit the ground when fully compressed. Then do the rest with the spring perch adjustment. If there is no tender spring, that will likely mean having almost zero preload on the spring to get the most droop possible.
Unfortunately, some of these kits just don't have much available suspension travel, so you're stuck in that case with not being able to do much!