Proportional Allocation

Hi All

I have a set of countries with a percentage allocated to them, for which they total a 100%.

I want to be able to change the percentage of one or more of them, but the total percentage must still add up to 100%.
The difference (+/-) created by the change in percentages should be proportionally split between the remaining countries.

I can do it manually of course (as is in the attachment), but was looking for some expert advice!
The attachment explains step by step.. the colors are there to guide you accordingly.

PS: The number of countries is endless, and i only posted an extract.

Many thanks
Sam