One more thing I think is worth mentioning is that you may not wait till all your 'craters' are repaired before retrofiting to another design. I mean, for example you retrofit 2 big ships. One is fully refitted the next turn and another is not. It still has some 'craters'. You may give this ship retrofit orders without waiting for all the components to be repaired. This will not reduce the cost but only time in case you have researched some new techs.
Sorry if this is off topic
