Sadly, that feature didn't make it into the game yet.
It estimates the BAC based on a 120lb female and 160lb male. (This is on the lower end of estimates on purpose as to not underestimate BAC)
The future release of the game will have the option to enter height, weight, and age in order to make the estimation more accurate.
If you have previously purchased the game, the update will be pushed to you automatically.
Thanks for taking a look. I hope you enjoy!