You need to set some options within the inputbox properties in tab restrictions. There you enabled the options:
entry-restriction --> "only predefined values"
valuelist --> "predefined values as dropdown"
predefined value --> "listed values"
and you need to set the "listed values" per expression, maybe by a valuelist($(=SomeLogicToRestrictTheList)) which will be restricted on the already choosen values from the other variables. Then without you won't be able to avoid a sum over these variables which is greater then 100%. Such logic won't be easy and you will need some trial & error.