One more question.
At start how did you get 64 remainder?From what value did you divide?
Roman Black initialize his accumulator with 100256uS on power up.
If you continually subtract 256 from 1,000,000 until the value is less than 256 you get 64. This can be done in excel as mod(number,256) or number-INT(number/256)*256. The number you start from doesn't really matter as the error will only happen on the first time period. For simplicity I started from 1,000,000.