in the SimBiology Model Analyzer App you add a step to your program to define and calculate an observable.
This new observable could be defined like this:
with unit = dimensionless.
It is important to use ./ instead of / because this new observable will be calculated after the simulation using the resulting time-dependent vectors x and y.
Alternatively, you could also define a repeated assignment inside your model with the same equation as above. In this case, you will have to define fraction in your model as a non-constant parameter. fraction is then calculated at each simulation step like any other non-constant model quantity.