Although am not sure what is your goal; here I go . . . 

You gave the example of using cellulase and then using glucose as the standard. In theory this looks fine, but in practice, unless you have a way of separating the glucose from all other sugars [monomers, oligos and polymers] the Phenol will react with all sugars.

There is no easy, fast, cheap way of having and accurate measurement of sugars. Every sugar has a different extinction coefficient. Moreover adding Sulfuric Acid to the sugars is very harsh so one has to be careful in doing this in a very systematic way to try to reduce variances.

If what you want is to measure  the amount of sugars produced by depolymerizing enzymes you could use a nice trick I learned many years ago while in Trondheim, Norway. carry out the reaction inside a dialysis bag. Ensure you have good mixing. This way what you measure are the sugars present outside the bag.

Thanks Mr David. Actually i am working on biodegradation of agricultural waste using aspergillus species (niger & flavus). Then i have to do the phenol sulfuric carbohydrate assay which i'm not so sure what is the significant of this assay in my project.

However, it is great that u answer my question. Thanks! :)