To get precise intervals in a timer interrupt you can *add* an
       offset to TMR0 register.  There is no need to know how many
       cycles into the interrupt you are.

            movlw     -INTERVAL+3
            addwf     TMR0

       The next interrupt will occur precisely INTERVAL cycles from
       the current interrupt.  The offset of three compensates for the
       two cycle write loss plus one cycle that is lost in the
       read-modify-write operation of "addwf".

       Good luck,
       Jim