I can tell you this as a former Starbucks partner: I was reluctant to completely fill a personal tumbler (Food Safety Standards required that the customer keep the lid, and we not handle it) because if it wasn't a Starbucks tumbler, I was unsure exactly how much room the lid would need. Some lids fit high, some come in very low. A very full personal mug that happens to have a low-fitting lid will inevitably cause some splash, onto the customer's hands, car, clothing, or what have you. And that is a danger. It's a safety danger and a liability danger. So myself and my partners would always leave some room there for safety's sake, even if it wasn't already requested for room for cream/sugar/etc.Here are some things you can do:If you're in-store, just hand them your mug and tell them you want it full. Then see how full it is and what they charge you. *Generally* we'd always charge for a grande if the tumbler was somewhere between a16-20oz capacity. And still fill it fairly full.If you're in the drive-thru, tell them you have a 20oz personal mug that you want FULL of coffee. Check it at the window when they hand it back to you, and feel free to ask them to top it off if it's not full enough.Order a venti. If your mug isn't returned to you full, ask them to top it off. A good Starbucks partner will have no problem with this.Do whatever you feel like, and let them know if it's not done the way you want it. Seriously. No sarcasm here. I'd have been glad to do this and would have preferred my customers tell me if things weren't to their liking, so I would have a chance to solve the problems and avoid the unhappy feelings a customer might walk away with.Good luck! Thanks for saving some paper cups!