Hello, fellow Blane Romancer here, I find it's better (and also classier) to prioritize Relationship over Romance, since the latter doesn't count for the former improving (honestly I feel like it's the other way around). It's pretty much on the nose: just be overly kind, don't fight with him, be diplomatic.
I'd say that you're safe to flirt with him from 20% and up, or when he can "sort of tolerate you".
You also need to grind encounters with him, eg go to the Training Room (where you can then choose to encounter either A, B, or N) and then right after go to the media room. (After a few attempts, I managed to milk 30% from these interactions!!!)
Happy rizzing, soldier! Hope I could help