Project Resource Rates

Hello

we're using EPM 2010 and soon 2013 to manage our clients' projects.

we are using a centralised resource pool and people will enter Timesheet for project tracking.

one particularity is that every project is having a specific agreement on a rate for resources.

I know that we have 5 rate tables in EPM but I see this as a limitation because someone in his life will surely be assigned to more than 5 project and starting to play with talbles AND effective dates leads to management nightmares.

so my question is : do you see a way we can edit the resource rate per project?

do you see any other alternative for our use case?

regards

Jérome