Is battery consumption linear? I am benchmarking an app using the batteryhistorian. 
The result is a chart where every percentile is a "tick".

After I've measured the battery consumption
and let the battery drain to half, I would like to be able to save time
by making  a change and restarting a measurment without having charged 
the deviced back to full.

Is it a fair approximation to assume that a drop from 85% to 84% is the same
as a drop from say 64% to 63% when benchmarking an application?
Li Ion battery discharge is not strictly linear, (unlike a capacitor ). This ensures available power is utilised till the end of battery charge





But it is fairly linear, more so at lower loads as can be seen from this figure, where C-rate or charge rate is the discharge/ charge rate





Assuming your app is not a heavy power hog, it is reasonable to extraploate discharge. But, since the discharge isn't strictly linear, it would give better results if


Battery drop interval is more , say 5 to 10% and not 1% as proposed
Battery drop is not measured below 30% of battery ( to avoid greying into non linear portion of curve)
Measuring at 1% drop inherently introduces rounding off errors- if your phone has two digit accuracy, anything between 75.90 to 75.99 would show 76% . In the worst case, the error could be doubled (it won't fall to 75% till it goes below 75.90). Which means a 20% error, and that can be reduced to 2% over a 10% drop 
Avoid measuring when the battery is fully charged to reduce errors. Measure at about 85% for the first reading and around 50% (or higher ) for second reading after tweaking your app