As a former bartender (not at Disney) I never expected a percentage unless food was ordered. $1 per drink was always standard, anything extra was very much appreciated. Yes, I think for a beer or a "this-and-that" drink (vodka & whatever, rum & whatever, gin & whatever, etc.) a dollar is standard and sufficient. For a frozen or muddled drink, you might go higher. For frame of reference, this was the late '90s to mid 2000's.
That said, as a bartender, I tend to tip extra, and at WDW, I tend to tip even more, within reason. I pretty much always tip $5 on the first round (of one or two drinks.) And then $2 on the second drink (or $3 for two drinks) and then $1 per drink for the rest of the session (or until I fall off my barstool.) I have a few favorites at WDW who I will tip more than that.
The variables: how attentive a bartender is. If they don't chat you up at all, they'll get no more than standard basic tip, especially if they are not busy. If they go anywhere from basic friendly chat all the way up to fully entertaining you while still keeping track of all the other customers and keeping up with their bar (like I used to do) then I'm more impressed and more inclined to tip higher and come back. I've said this before and I'll say it again, Julie at the Tune-In Lounge is an excellent representative of Disney and of excellent tending bar. It's not just pouring drinks. Your brain needs to be constantly managing every chair.
And although this can be an ethically gray area, I will tip extra for stronger drinks.
That being said, overpouring = stealing. I used to have people ask for less ice or a tall glass because they thought they would get more alcohol. That's not how it works. A pour is what the recipe calls for, and pouring extra is stealing from the owner of the bar who owns the alcohol. Alcohol is like liquid money. Now, you don't know what "store policy" is. I've had bar owners explicitly instruct me to give free shots (nothing expensive) if someone was sitting and ordering 3 or more drinks, or if you wanted to get some customers to start talking to each other (to get them to stay longer.) OTOH, I've had bar owners watch from cameras to make sure your "4 count" was not a 4 1/2 count. Many bars today use pre-measuring devices like jiggers or special pour spouts that measure an ounce. In those cases it's tough for a bartender to get away with anything.
The point of all that is, if you get something "strong" or "extra," appreciate it. If you don't, you have no business expecting it or asking for it unless you'd like to pay for a double.