RE: GP 2010 setup Employee HealthCare Benefit to be 50% of total Premium
If I understand you question correctly, it seems that you have at least two possibilities.
First, when the amount changes mid-year use the "Master" screen "Microsoft Dynamics GP >> Tools >> Setup >> Human Resources >> Benefits and Deductions >> Health Insurance" and roll down the changes.
Second, you could setup the deduction as a Miscellaneous Benefit and have the Employer side be 100% of the deduction and the employee side be a fixed amount. Again when it changes mid-year go to the "Master" screen "Microsoft Dynamics GP >> Tools >> Setup >> Human Resources >> Benefits and Deductions >> Miscellaneous Benefits" and roll down the changes.
I would suspect that the first would be more desirable, and there is probably other ways of doing this that I did not think of while typing this reply.
Hope this helps,