Okay, there are actually 2 "editions" of koni yellow shocks. The normal ones have 3 perches, with the middle being "neutral height". Each perch is 15mm up or down from neutral so if you set your springs at the LOWER perch, that's an additional 15mm lower.

Then there's the Neuspeed Edition Koni Yellows that have 5 perches. Similar idea...the middle perch is neutral, but the thing now is, the highest perch is 15mm up, the lowest perch is 15mm lower. So, with the Neuspeed editions, you get an extra perch that lowers 7.5mm then one more to get 15mm. The Neuspeeds are harder to find, and usually cost a little more.

By the way 15mm is bout 0.59" which is a lot. Keep in mind that any aftermarket suspension will settle..meaning that the height it rests at when you first install is higher than it'll end up being...how much it settles depends on what parts you have, but people typically say "give it another 0.25inch"