if 12.5 is 5% of 250, then what is the 5% of 262.5?
262.5 = 250 + 250*(1/2)*(1/10) so
5% of 262.5 = 12.5 + 12.5*(1/2)*(1/10) = 12.5 + 6.25*(1/10) = 12.5 + 0.625 = 13.125
Is that what you are looking for?
.