High speed counter/ctbl function

[baptie 0]

I am using a CP1L plc and have two high speed counters used for pulse output to servo and main drive. On the servo I am attempting to use CTBL function. I have set up the register with 2 interrupts. One opens a solenoid and one closes it. The problem is that the position alters every time so I cannot hard code two values. I have a sweeping head with solenoid fitted and proximity sensor. When the sensor picks up target I fire the value to first interrupt figure in ctbl and when trailing edge has been picked up I fire this value for the second interrupt value. My query is, is the above method acceptable as I don't seem to be getting the interrupts to run. Can anyone shed some light on this please as I am not 100% on the ctbl function?