To calculate the F factor you use change in absorbance, the spectrophotometer should be blanked on the reagent blank and all absorbances should be read against thi at 510 nm.

F factor is calculated as follows:

100 μg of glucose (as per the supplied standard when using 0.1 mL) divided by the GOPOD absorbance value obtained for 100 μ g of glucose, read against the reagent blank at 510nm.

Please see below for an example calculation using an absorbance of 1.050.