Hello Jim,
I think there is still some confusion.
By your own figures the caravans MPTLM is 1300Kg and the MIRO is 1103Kg leaving a payload of 197Kg. You cannot arbitrarily increase the pay load because it will exceed the MTPLM for the caravan which is a legal maximum.
So you have 197 not 200Kg. I know that 3kg is not much different but you have to also consider that the caravan MIRO may be higher than you think. This can be down to a combination of tolerances on manufactured items, and the amount of moisture the caravan may be loaded with and any OEM extras.
As Lutz correctly states, the towing ratio that is used always uses the kerbweight of the car and the MTPLM of the trailer, so it is always a theoretical figure, but it is a base line to allow comparisons. This means the normally quoted towing ratio is independent of how you actually load the car and caravan, but it does describe the worst case scenario.
Whether by choice or just good luck, you have managed to put together an outfit that produces a ratio of 87% not 83%as you originally stated.
With reference to my earlier post suggesting putting the bikes in the caravan, you may still do that provided the caravans MTPLM is not exceeded.
Incidentally I sense from your opening post that you did not wish to increase the caravan's weight. On the whole that is a sensible approach, but if it was anything to do with often quoted 85% ratio, then rest assured, it is not a legal requirement, and if you have a good towing characteristic at 83% (your figure) then the additional weight of the bikes is unlikely to take you into unstable territory.