Depending on the code you have you will have to address this soon:
If the input box will not accept more than the maximum credits or less then the minimum credits
and there is a check calculation which verifies the no of converted bitcoin is half the number of credits
then withdrawal will be impossible when the exchange rate gets to less than 0.2500
OK, So I have managed to confirm this through experiments in the past few days.
The withdrawal form will not let you withdraw over 100,000 credits.
The withdrawal form will convert the payout to satoshi and will not let you withdraw less than 25,000 satoshi (assumed to be calculated half of the minimum balance)
Therefor the premise is true: withdrawal will be impossible when the exchange rate gets to less than 0.2500 since you will need more than 100,000 credits to achieve the minimum withdrawal of 25,000 satoshi.