In my mind what Spock calls Kirk works something like
- "Captain" : professional detachment. We are At Work. I am in control of my emotions. I'm being so normal right now
- "Jim" : personal fondness. Open affection. I consider you a dear friend and companion outside of your role on this ship
- "Captain" : profession of undying loyalty and devotion. I would follow you anywhere. My one true place in this universe is by your side. Unbelievably Horny
Let's not forget, additionally:
- "Captain" : I have been calling you this for so long that it's become a pet name. Term of endearment. Playful by Vulcan standards especially if off-duty or, like, not even currently holding that rank
- "My Captain" : Somehow Even Hornier
And on the other side of the equation we can just rank every time Kirk says "Spock" on a scale of how wistfully he's sighing it
Slight correction: he also says "Mr Spock" like it's either a formality or like he's writing it in glitter gel pen inside a heart in his diary several times trying to figure out how taking his last name would work with Vulcan naming conventions