For a cell that contains a percentage, try the following format:

#,##0.00%;-#,##0.00%;;@

If the value is either positive or negative, it will display. If the value
is 0.00%, then it will not display. This is the style that I use in my
workbooks when I want to hide values or formulas that are (or evaluate to)
exactly 0 (zero).

For a formula that returns a value (not a string), like the following:

K29: =IF(H270,R27+C12*R29+C11,0)

then make the IF statement give a value of 0, if the IF condition is FALSE.
Then format the cell as above with a blank part for the negative
specification to cause the 0 value to not be shown.

(I changed the commas in your example formula to "+" signs. I still don't
understand what your formula is doing!)
--
Regards,
Bill Renaud