It depends on how many points of measurement you have. I'm not certain as to how exactly you come to a mean value when incorporating fasting values with post prandial ones, but know for a fact that there are alot of papers on it. Many of them incorporate formulas to measure the area under a curve, but I don't recall the details. Read this paper and see if their protocol fits what you're after.
Efficacy and tolerability of the dipeptidyl peptidase-4
inhibitor sitagliptin as monotherapy over 12 weeks in
patients with type 2 diabetes by Scott, Wu, and Stein.
Good luck!