[charrisTTI]

Using the port b interrupts will help.

Remove all of the extraneous calculations from the interrupt routine. These can be done in the main line code. ( or code redesigned so that it can be done there )

When accessing interrupt variables in main line code, be sure to disable/enable the interrupt. Otherwise, the data can change in the middle of your code. Keep the disable/enable short.

In unsigned math, overflow of the counter is not a problem, trying to measure a time period longer than the period of the counter is a problem. The difference between two unsigned counter values will be correct even if the first value is larger than the second value.