Allow only a numeric value to be input into a cell

I have a cell formatted as numeric . Its used in formulas and works great. but if the cell is loaded with a space then it messes up the formula. I need to only allow 0-999 in the cell.

excel 2007 .

I did a data validation and restricted it to 0-999. But I dont know how to change an amount to an empty cell. I can change it to zero but it would like the original empty.

So this question is basically how can I allow spaces, 0-999 and still have a numeric calculation work?

Thanks.