In the fork choice document we specify a stickiness of 1 epoch for justified epochs.

Let justified_head be the descendant of finalized_head with the highest epoch that has been justified for at least 1 epoch

This parameter of "1 epoch" should be a configurable constant -- MIN_EPOCHS_TO_USE_IN_FORK_CHOICE (placeholder name).

It could also be defined in terms of slots to get fractional epochs