tipping butler

I just read Kuki's article on butler service and was wondering about tipping.
We will be in a Sky Suite 1211 on the Mercury this summer and I have no experience with butlers!
Do we tip him every time he brings us something or at the end of the cruise ? We have always tipped for room service, bartenders etc at the time of service.
Thanks for some advice. Emma

[Kuki]

Suggested tipping is the same as for a cabin steward... which actually surprised me. Our butler did so much, I thought he almost gave the cabin steward the week off.

We did tip him above the suggested $3.5-0/per person/per day.. at the end of the cruise.

He tipped me nothing for the day I worked.

Thank you, Emma

I tiipped him 50% more than the suggested amount. He earned every bit of it and did much more than the cabin steward. We had a party in our suite and he stayed and served the food and poured the drinks. Were on the Infinity and his name was John.
Don