OAB Imputed Income $50,000 non-Exempt Set-up

 Hi
We have OAB implemented and Imputed plan in OAB. Basic life is subjected to Imputed income. (we are not using fast formula - we have pay value defined in imputed plan set-up
I have two questions.
1) where is the 50,000 tax exempt amount is defined in the system
2) we have variable coverage and rate calculation for Imputed GTL. in the definition , calculation method is flat amount but while doing calculation, it calculates as
   (coverage - 50000) * rate (i.e.flat amount .0005)
 flat amount should be constant instead of the above calculation

Thanks
makarand