First thing to remember is that tipping is never required on Amtrak. Also, on-board personnel are not paid sub-minimum wage (as many land-based service/restaurant employees are). So, keep in mind that your tip compensates the Amtrak employee on top of their already competitive wage.
That said, most people find it appropriate to tip at mealtimes to the same level as you would in a traditional restaurant. As a sleeping car passenger, you won't get a bill for your meals (unless you purchase an alcoholic beverage). But you can leave your tip (in cash) on the table when you leave. (I try to take along small bills for this purpose)
I wouldn't worry about the "percentage" since Amtrak dining car meal prices are somewhat inflated. I generally tip a dollar or two per person for good service, or more for great service. I have also declined to tip when service was horrible, which sadly happens all too often.
I would never tip a sleeping car attendant (SCA) at the beginning of the trip since you cannot know at that point whether they will be deserving of one. I understand that some people tip up front as a way to grease the skids. But an SCA is required to serve all passengers equally well regardless of financial incentive.
As far as how much to tip an SCA after the trip, theories vary widely on this. For good service, some people tip $10 or even $20 per night. Frankly it seems a bit spendy to drop an extra $40 on someone just for doing their job. But if the spirit moves...then go for it. I'm more in the $10/flat rate range for good service, a bit more for exceptional service. I've also declined to tip when service was far below standards.