"Is the filter blockage indicator the optimum change point for the element?"
No, this is not necessarily the best indication, although it is better than ignoring the indicator. For example, keep in mind that a rupture in the filter media will not be indicated by an increase in pressure drop. If a cleanliness target is set, and this is regularly monitored by a particle counting program, then change the filter when the cleanliness target is exceeded.
However, ensure that this is actually the cause of the problem by taking upstream and downstream counts, as well as ensuring the target has not been exceeded because of other ingression sources such as a failed breather.
Also, consider monitoring the differential pressure rather than just watching the indicator, as experience gained with particle counting will show what pressure is the optimum change point.