I meant we don’t need your initial “90%” rule neither the rule of counting only the 2 ways certifications.
We could add the 4) recommandation, maybe under the form of a:
It is a good practice to certify the people who certify you, both for courtesy and web of trust security.
Or something alike. 
Yes it would be handled a similar way: each time frame (i.e. when a new valid block is received) we compute for each member if he is a sentry1 or not.
It would not be that costly because we do not need to make the complete computation of the WoT each time, we just need to do it on members impacted by:
- broken links (just expired certifications)
- new links (new certifications)
- new members
- gone members
[1] that’s how I call the members against which we test the WoT rule