If they use half the rooms/bathrooms and 2/3rds of the utilities then the suggested pay scheme sounds pretty fair.
Ultimately, there’s no one absolute “fair” price, just one that seems reasonable and doesn’t involve roommate warfare and hatred (nor of them determining that they can walk over you). The only suggestion beyond their scheme that I might suggest is figuring out the total square footage of the unit and of the communal areas and allocating by that. For example, if the total area of the unit is 2000 square feet (sf), their bedroom and bathroom is 300 sf, yours is 400, and the communal area is the remaining 1300, then you would pay ((400 + (1/3) * 1300) / 2000) * RENT and they would pay ((300 + (2/3) * 1300) / 2000) * RENT. Since RENT is $1300 per month, in this example it would be $541 for you and $759 for them.