What's an appropriate amount to tip in this circumstance?

A tip is not a gift, however the rest of his answer is right on. IF servers were paid a fair amount, then most could not afford the price of the meals because they would be raised higher than acceptable or affordable to make up for paying the servers wages. Servers get around 2.13 an hour, higher in some states I'm sure. 20% is always acceptable, but if server is good and pleasant and just attentive enough, I always leave a little over 20%. I would've left 5 on your bill, but I'm sure she was not disappointed with that amount. I figure tip by rounding up to whole dollar and X 20%.

A 20 % tip is very reasonable, especially considering others don't apply that percentage regardless of the great service given. I think it all depends on how long your sitting at the table for. Servers are often limited to tables so if you have a customer who is having water and a meal that adds up to a 12 dollar total and decides to sit there reading or do work for 2 hours,I personally think the 20 percent tip needs to be ignored. The guest should take in consideration that it's costing the server the opportunity of making money off serving others being that the table is being taken for a long period of time. Example: you left $3.75. Say you stayed for 2 hours. Divide that by tip by the hour and in all reality their making less then 2 dollars an hour. it can play on the servers favor where a guest is there for 20 minutes from being in a hurry and they just made cost 4 bucks by not doing much. So I guess it all depends on your specific incident :)

A tip is gratuity. So it's a gift, however I will usually leave at the very least 15%. Even if the service was subpar. How ever often times I leave 20% and even round up. So I believe you should tip how you feel, If you want to leave 30% i'm sure the waitress won't mind. But If you leave anything less than 15% I hope it's because you were given horrible, unprofessional service.

It's not. You're supposed to get paid more for the more work you do. You don't. Nothing's going to make that situation fair, at its bare essence, unfortunately. :(

I know you mean - usually if somebody brings me food and drink - even at a bar I tip the more of 20% or $5.

If 15% is less than $5, I leave $5.

Stop worrying. $3.75 is still nice. Next time leave 5.

That sounds like a good amount to me.

No, it's not too little. She already gets paid for her job, a tip is only if she did something special and you already left a fair bit of it.