Negative data bars in conditional formatting

I'm creating calibration result forms for a range of electronic equipments and I'd like to use the Conditional Formatting Data Bars as a visual cue to the results.

As an example A result maybe 20dB+-5dB, giving me a range of 15 - 25dB

Thus A1=15 C1=25
B1 will be my result with 20 being the ideal.
If the result is between 20 and 25 the data bar can indicate that result.
But if the result is between 20 and 15 (in other words negative of the ideal ) then I don't seem able to get it to work.
The Data Bar seems to only work with ACTUAL negative figures...or am I wrong?