preventing the non-numbers from even showing would've been the ideal result, but if I can't do that, then having the edit box automatically reset to a default value after a certain time period will work well enough. As to using a drop-down combo box, while it would work from an operational point, I'm not sure how well it would do from a visual point, as I've got 8 of these already set up...but it's something I can consider along with the responses from everyone else.
