How is the Per diem rate calculated in the system after having rules of deductions and 75% rule.

Summary: How the per diem rates are calculated when we have Deductions enabled and 75%first day and last day implemented as well.

How does the system calculates only the dinner rate or only breakfast and lunch rate if deductions are opted , We are getting different amounts randomly for all the combinations. For examble for a rate of $86 having 75% rule would be 64.50 but if only the dinner rate is included it gives 19.5 and if only lunch is include then its 6.50 and for only breakfast it's 5.50 but when breakfast and lunch are combined then its giving $28.50 as the total amount for the two meals on first day.Is there a calcution method for the same ?