So your saying once the interupt is triggers, the timer can be used to measure directly the length of time it is low?
not directly but you have to take samples so that you know when the low - high transition occured.
(never used the timers before) I'm a newbie as far as the code goes, I wouldn't know where to start.
Read the data sheet, play with the timers and timer interrupts till you are happy with it. Then use your imagination to do what you want to do, don't limit yourself to existing code. There is many ways of doing it.