Everyones carb ratio will be different, and if two people share the same carb ratio at one time of day (e.g. lunchtime) they may not share the same carb ratio at other times of the day. There is no formula that will "fit all", the only way to work it all out is to test and then repeat testing until you find numbers that suit you. To avoid different GI food interfering with your calculations you could try eating the same food at different times of the day to see how it differs. There are a number of reasons your carb ratio may change at different times. I had assumed that it was injected basal insulin wearing off towards the end of it's life and the next basal injection not having quite kicked in yet, and that might be right. But since my daughter has gone onto the pump we've realised that her basal insulin needs fluctuate during a 24 hour period (stress, growth hormones, exercise, excitement etc causes this) but also her carb ratios still vary too. I think due to the stress, hormones etc this might also cause periods of more or less insulin resistance during a 24 hour period and perhaps that's why the carb ratio still varies. Again the types of food will affect everyone differently so to work that out it means more testing and evaluating the results. I'm afraid there never seems to be an easy answer with diabetes!