Your question is unclear as you missed two cases

What do you want when
1) U51<0 (negative value
1) U51 is not blank and not numeric. For example, U51 contains tex

Besides, your formula is wrong because you said the result is ok when
1) U51 greater than zero; an
2) U51 is blank

But your formula also deals with value smaller than zero
You should use U510 instead of U51<0

Please give more details. Thanks