I know you can say "I bet/raise the Pot" (and let the dealer work out the number of chips) but can you specify a fraction/percentage and let the dealer work it out ?
Yes, although it is kind of a vague way of asking how much the pot is. You can always ask a dealer how much is in the pot and then size your bet anywhere from to minimum bet of a game to the pot size. When a player says I bet the pot he is betting the pot, and has a right to know what is in the pot. Same thing if you want to bet half the pot, or any part of the pot. In the nitty gritty the dealer is only obligated to give you a correct pot size, the player really needs to figure out what their actual bet is going to bet. You might ask what 26.54% of that works out to, but your going to get a lot of shoulder shrugging.