Interesting question posed by rmz80, which I'll try to answer to the best of my knowledge as I understand things.
Firstly taking a B/G reading before eating and then agan 2 hours lates (sometime it's also take at hourly intervals) will show just how well, or otherwise the individual is at coping with that particular combination of food/drink items in processing the resultant Glucose that when the the meal is broken down by the digestion process gives a rise in B/G levels.
We arn't all quite the same and some items might produce a greater rise in B/G levels (Spike) in one individual, more so than the same meal being consumed by another individual. This is why over time we each get to know what we can, and cannot eat.
The Glycaemic Index was first established by measuring the resultant B/G responce over a time period of a SINGLE type of food item and relates to consuming 100gms or 100ml of that item. This is purely a reference guide, as it's not really a practical proposition to eat 100gms of say 'Carrots' on the dinner plate at one sitting without experiencing some possible discomfort !!!!
For this reason real portions were then considered rather than 100g/100ml of the item, portions such as you would eat on a dinner plate, and still based on the Gi system the figures were ammended and become the Gl (Glycaemic Load) which is in reality a better indication of the effect of the food/drink item.
Combining a High Gl food item with a Low Gl food item on the same dinner plate will result in the (Average) value being around the mid position of the Gl initial values. The same can be said for multiple combinations of food/drink items which as you will probably agree would be impossible to accurately list due to all the countless combinations that this would create.
The simple answer is to eat according to your meter.
