First, got to say I am not a doctor so this is not a "professional opinion"
First, the most important part. If you stick closely to Dr McDougall's diet recommendations you are virtually certain to never become diabetic unless you have a very rare autoimmune condition. A low fat, plant strong diet will eliminate existing insulin resistance and prevent its future development. And that is basically the cure/preventions of type 2 diabetes!
Both readings are likely fairly accurate but they tell you different things. The afternoon one, gives you a very rough idea of how you process the blood sugar from a meal. From what I have read, the best marker for that and the overall best indicator of risk of future diabetes is really your blood sugar level approximately 1 hour after you finish eating. That is, you are really looking for the highest level it goes to. I am in US and so am converting from my measure but the research I read says you want to be under 7.7 one hour after a meal. The morning one, before eating is called fasting blood sugar and medical opinions vary as to what is safe and what is ideal but probably 4.5 to 5 is what you really want. Also, the one hour after meal one should return to the fasting level pretty quickly after that, within another hour or two.
I think medical doctors in the US say your fasting should be under about 5.5