Yes, it's a long way from an exact science, but good enough to help me to keep things in bounds. For example, had a virtually zero carb lunch today - my own made vegetable bhaji with some ham and coleslaw - expecting post lunch reading to be low (for me). And at 5.3 it was, so I treated myself to a biscuit (17g carbohydrate) with a cup of tea to make sure I do not drop below 5.0 before my tea which again will be very low carb. I'm thinking that I will be in the low 5's a couple of hours after tea ( biscuit will have worn off and tea will be adding very little) so I will check to find out. All jolly good fun, but seriously, if I can get a decent picture of what is going on and develop decent hypo and hyper awareness then I hope I can get this thing under control without having to think about it too much.
Also when things have gone to **** then I have been able to trace it back to a meal where my carb estimate was probably in error.