Set up an auto-enter field option to enter 0 into this field and it will initially start on zero like you have specified.
wow, that was a superfast reply,
i tried it, but the first value in the drop down list is still the top value (in this case -20).
I was so fast, that I didn't think that one through all the way. The 0 should be hilighted, but the entire value list will still appear as a single list as you describe. That's pretty much all you can do with a single value list. You may be able to set something else up that works better with buttons to click on a number scale or to set up two value lists, one for negative values and one for positive. But none of these will be exactly what you have requested here.
mmm ill have to look at some sort of pop up sliders or something, at least i know there is nothing built in. thanks foor the help
You might use a text field or a portal where the values are listed vertically and then clicking a value selects it and enters that value into your field.