Hi, If you're interested in the ratio of changes, then the technique you suggested is perfectly fine. Measurement errors, If needed, may be calculated for your ratio technique as followed: Define - X as initial value and Y as final value, thus Z=Y/X is the value you're interested of. According to the least squares technique, measurement error range (Delta(Z))=Sqrt((delta(Y)/X)^2+(Y/delta(X)^2)^2), where delta(Y) and delta(X) are errors within X,Y. If you need some clarification, post again. Good luck.