There's geometric models for understanding virtual centres, and from these roll centre, and bump steer.
Put very simply if you draw a line from where the upper and lower ball joints pivot through the pivot for the upper and lower arms, where these two lines intersect is the virtual centre.
To have no bumpsteer at a particular suspension height the steering linkage from the steering arm to the rack should also point at this virtual centre.
If you've moved the rack down 20mm my brain is telling me you'd need to move the steering arm down more than 20mm to compensate.
Project binky had a bit where they delved into bumpsteer which is probably worth a look.
Here's some stuff on finding virtual centres called virtual pivots in this article,
http://www.autospeed.com/cms/article.ht ... n&A=112911