I wouldn't tip for anything I hadn't received. That would be ridiculous.
But if I got and drank two rounds of drinks and I got and ate an appetizer, then even though I didn't get the remainder of my order I would tip for what I had imbibed/consumed.
IF I only received part of my order and IF I felt compelled to leave anything, it would not be more than minimum wage. I would not tip a percentage.
Since minimum tipped wage is $2.13 per hour with the expectation that a tip would bring it up to the federal minimum wage of $7.25 per hour, I would calculate how long the server worked for the drinks and appetizers and then calculate what I would leave from there.To make minimum wage, a customer should supplement a server's $2.13 by $5.06 an hour.
So, for instance, if I was served for half an hour, I would give the server $2.56. This would be half the make-up to get the server to minimum wage. Also knowing that the server is making more than minimum wage for that half hour (since the server was busy with other tables, so would also be receiving minimum wage for the same hour from other tables), I would not feel guilty at all.
If a server fails to provide adequate service for a customer, they should not be entitled to more than minimum wage.
And my standard tipping policy is 20% for adequate (but full meal) service and more for good or excellent service.
However, poor service would get minimum wage, not a percentage. And in a state where servers already make minimum wage, I would not feel obligated to leave a thing.