As others have said, service and how much you utilize butler, Concierge and room steward services should drive how much you tip.
The bigger discussion is that many people who are in the Haven for the first time don't really know what their options are in terms of services - which is why guides like @Sthrngary's are a great resource.
In my opinion, the greatest benefit of the Haven and butler service is being able to design a personalized vacation. Once you are aware of all of the services offered/available, you can pick and choose what works for you and then tip accordingly.
For example, folks talk alot about the daily snacks. Personally, the first thing I tell my butler is NO snacks (and that the casino or NCL/Captain may also want to send something but don't bring it.) I travel solo and am a very light, picky eater and all of those snacks would go to waste on me. On the other hand, others may enjoy tables stacked with snack & treats and that's great, too. I utilize the butler for packing and unpacking - many would say they'd never do that - also ok! I eat most of my meals in my cabin - but I only eat one full meal a day and it's usually midday (so no dinner.) Other folks may have their meals delivered but eat breakfast, lunch and dinner every single day. And so on with all the many other services offered - it varies widely from person to person!
That is why it is very hard for anyone to give an "appropriate" tip amount (and what leads to all the debates lol). My recommendation is that you read a guide like Gary's (and others) on this forum to learn ALL the possibilities, decide what services you want to use to design YOUR perfect vacation, discuss your preferences with your butler and then tip based on that.
Have a great cruise!