hi,

I received the same letter as well. Basically, the letter does mention the formula to come out with the utilisation rate:
Average Daily Flexi Account Balance (RM) = Sum of Daily Flexi account Balance / number of days in month

Utilisation Rate of the month (%) = Average Daily Flexi Account Balance / Current Facility Limit x 100%

In summary, the 'account balance' means how much you owe to the bank. As long as you still owe them 70% or more, then they won't charge you for flexi charges