The lower control arm is where the spring sits, one side at the support joint in the wheel carrier, the other at the subframe towards the differential. One side or the other is taken down to get the spring out (thought I've talked to one person that managed to keep both sides attached). The bolt thru the bushing or joint is removed, the arm pulled down, the spring taken out.
It in itself is unlikey to signficantly affect ride height, but the arm should be tightened with the axle horiztonal. I estimate about 1/4" lower from tightening everything with the axle horiztonal (including caster arm).
If your car is about 1/2" higher its quite possibly the HD Bilstein struts/shocks. If you really wanted it lower and keep the HD ride, you'd probably have to go with Bilstein HD Sports with corresponding shorter Eibach or sportline springs.