Minus .5 z value, how?

[acemark41]

Yeah, it's me again, hi. I'm having trouble with z values now, Do'h! I had a look at several excellent tutorials elsewhere on the site but can't find anything to help me out. I want to drop some crates just a fraction because I'm putting them on some uneven terrain and I can see under them (which I think looks really naff). Now I know about the two z value boxes ( 1st one being metres & 2nd one parts of metre) and that the second (fractions) box won't take a - so how do I drop something (my crates) by, say -10cm???? I tried putting -0 in the first box but it changes it back to 0. Any pointers folks?
Cheers.