If you need the cell to display as empty but evaluate as less than other (positive) numbers, try using =IF(condition,something,0) instead of =IF(condition,something,""), then apply a custom format to it of 0;-0; (zero semicolon minus-zero semicolon). That will show any number normally unless it's zero, when it will show as blank.

Hope that makes sense and is maybe of some help.

ps Welcome to the forum.