For now, the Mirai's H2 competition means the 2015 Hyundai Tucson Fuel Cell. It can carry 5.64 kg of hydrogen and has a range of 265 miles. If we do the division there (265/5.64) we get 47 miles per gallon equivalent (MPGe). The DOE says that the miles/kg values are 49 combined, 48 in the city and 50 on the highway while Hyundai lists the official MPGe estimates as 50 combined, 49 city and 51 highway. The simple math gets us pretty close to these official numbers.
Or take the 2014 Honda FCX Clarity. With a range of 231 miles and a max of 3.92 kg of hydrogen on board, division gets us to 59 MPGe. Officially, it's rated at 59 miles per kg (combined), with 58 in the city and 60 on the highway. In other words, simple math is a reliable way to calculate rough MPGe.
So, we know that the Mirai can hold five kilograms of hydrogen on board and that the car has a 300-mile range. We don't even need a calculator to figure out that the Mirai is looking at 60 MPGe. Interestingly, that might be what the next-gen Prius will get.