This figure was calculated in 2015 by researchers from the University of Geneva on behalf of the Federal Office for the Environment (FOEN) and is based on the total Swiss CO₂ budget, which was then distributed evenly among all inhabitants by 2100.
Since this budget has already been exceeded for several years, the effective CO₂ budget for the future is even lower. As a general rule, the later the target of net zero emissions is reached, the higher the negative emissions will need to be later (see: What does "net zero emissions" mean?).
Sources: Dao, et al. (2015)