Hi,

I generally tip 15-17% ( rounded up to the nearest dollar)for service ( 20% for good service) I have not tipped less than that even if the service has been below what you might expect but have not had too many of those experiences.
If I only have a main course with a soft drink ( costing $20 or so ) I have tipped $5
What is the accepted wisdom at buffets- I generally give $2-3 per person ( maybe a $1 or 2 more if the waiter/waitress comes round often with coffee or oj refills without me having to grab their attention)- I do like orange juice in the morning.

I use the pre tax amount to base the tip on.

Regards

TBS