Full Tilt Poker has added a new option that you can find in the Options dialog which allows you to customize the Bet Buttons on the Bet Slider at the table.
You can choose the bet sizes you want for No Limit and Pot Limit games, both Pre-Flop and Post-Flop. In the Options dialog, select Table Options then Custom Bet Buttons. Choose the multipliers that suit your game from the dropdown menus or by typing directly into the boxes.
Pre-Flop bet sizes are in multiples of Big Blinds and can be set up to 6X for both No Limit and Pot Limit games. Post-Flop bet sizes are a percentage of the pot size and can be set up to 150% for No Limit games, and 100% for Pot Limit games.