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There is a rule for it: WP:HE. The short version is that popular English forms should be provided first; Greek text should be provided; the Greek text should always be glossed; the gloss should use the appropriate format (the scholarly transliteration formats for ancient or modern Greek as appropriate). There are iffy cases and some minor differences between systems, but for the most part just choose one and be consistent in the article and you should be fine.

In your case, it's a modern name and there's no difference between names from 1500 or 2000: accent marks and diaereses are included and everything else is left out. — LlywelynII 

Macron and breve
These are included in the Beta Code and here but there is no explanation for what they're supposed to do. — LlywelynII "