I got this to work with the cell to be tested as A2 and the target in
B2. I entered 75 in A2 and 80 as target in B2.

With the cell pointer on A2, click Format Conditional Formatting.
Set the first condition to "Formula Is" in the drop down, and then
enter =ABS(1-(A2/B2))<=0.15. This checks to see if the test cell is
within +-15% of the target. (I set the format to paint the cell
yellow). Then click the Add button and set condtion 2 to "Formula is"
and enter
=AND(ABS(1-(A2/B2))0.15,ABS(1-(A2/B2))<=0.2)
.... which checks to see if the target is greater than +/-15% but less
than +/-20%. I set the format to paint the cell blue under these
conditions.