Welcome to Cruise-Chat!
I go by the tipping guidelines that the cruise line has suggested like Dave noted. For example Carnival's are;
$ 3.50 Per Day Stateroom Services.
$ 5.50 Per Day Dining Room Services.
$ 1.00 Per Day Alternative Services: distributed to other kitchen and hotel service staff.
These are per person. I always give extra the last evening of the cruise to those that went above and beyond average service. Which is usually most of them. For room service I tip according to the size of the order and promptness, with $2.00 (2-$1.00 bills) usually being the minimum.
For porter service, the customary tip is $1 per bag or piece of luggage. I usually stick to that when dropping off luggage at a port. Now when disembarking and I use a porter it goes up. Especially if they get me to the Customs line quicker or have to go a long way to my car.