calculate a percent difference

percent difference between a known value A and a calculated value B is:

        |A - B|
% dif = ------- * 100%
           A

In our example, A = 12; B = 12-2*.001148. so their difference, A - B, is simply 2*.001148 and hence percent difference is:

        |2*.001148|
% dif = -----------  * 100% = 0.019 % 
           12

Less than 5% so the assumption that x was negligibly small is good.