Are there limits to the % discount versus valuation?

I am assuming you are referring to the forfeiture rate discount applied to the fair value of the grant for expensing purposes:

1) There is no set limit to the forfeiture rate discount
2) That being said, you do need to have some reason to believe that this rate is appropriate for your population, i.e., historical trends or expectations of a RIF, etc.
3) You will have to true up to actual vesting and forfeitures, so in the long run your expense is the same regardless of the forfeiture rate discount. The real difference between a high rate and a low rate is when you end up amortizing the expense, and how smooth over time that amortization is.
4) Your auditors will need to approve the forfeiture rate used in your expense calculations and will need to understand the reasoning used to determine the forfeiture rate

If my assumption that you are referring to the forfeiture rate discount is incorrect, please re-submit your question with further details.