KPIs in Docplex?

I want to add a KPI to calculate the percentage of occupancy of vehicles: the total travel time of occupied arcs (where the capacity of a vehicle is non-zero) over the total travel time.

How can I represent that using three variables: the $$x_{ij}$$ variable equal to $$1$$ if there is an arc between $$i$$ and $$j$$, the $$q_i$$ the capacity of vehicle after serving vertex $$i$$ and the $$b_i$$ the starting time of service for vertex $$i$$?

from docplex.mp.model import Model

mdl = Model(name='buses')
nbbus40 = mdl.integer_var(name='nbBus40')
nbbus30 = mdl.integer_var(name='nbBus30')

nbbus=nbbus30+nbbus40