Wine ratings go by point system and are based on 100.
A=90-100 (A+ = 97-100, A = 93-96, A- = 90-92; similar for other letters), B=80-89, C=70-79, D=60-69, F=50-59.
Just because someone doesn't pick 90 - 100% for their wine (everyone's pallet for wine differs) a 70 - 79 could be a crowd pleaser too. My husband is a great wine drinker and just likes to experiment with his own pallet. The way to test a couple of bottles of wine is to roll the wine around in your mouth to get the taste, spit it out and drink a glass of water and then do the same with the other bottle of wine. This way you get a clear taste of each.