> The best that I have been able to achieve is a variation of 2.5%

High or low ? IOW, are you missing pulses or counting noise ?

Reed switches ISTR are relatively bounce-free but it wouldn't
hurt to put an RC filter on the input to the ST. Values will depend
on the length of the pulses you want to keep/get rid of

You could use one of the timers as suggested or an INT input. The
first switch closure will set INTF. If you don't clear that flag for a time,
eg 100ms, that's effectively done the debouncing. For such a slow
pulse rate you could poll in s/w or use an interrupt + debounce delay

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