Is anybody aware of a jQuery Pro Rata Wheel? I checked the usual places but couldn't find anything.
There's a jQuery Calculation plugin that might be useful here as something to build from.
You can attach it to a group of form fields, with operations related to the set. You'd still have to work out the actual math operations yourself, but it will take care of all the update events and such to keep the "total" field up to date and so on.