Word of warning.... Be carefull using the saddle valve to punch your carto's. If you use too much pressure the carto will deform (it will oval or become egg shaped) where you punch. After you punch it roll it on a horizontal piece of glass and watch to see if it rolls smooth or not. Some tanks will leak or the carto might not fit at all, depending on how bad it's OOR.
I prefer to cut mine with a Dremel where the slot in the side is flush or very slightly below the floor of the tank, that way it acts as a sump and any juice in the tank will find it's way into the carto. If you have doubts about your measurements or your skills with a cutoff wheel on a Dremel it's better to cut high and lose a couple drops of juice then cut low and have it not work at all.