You have to calculate the necessary power to drive with for example 100km/h. Than you should assumed a ratio between the electric power and the ICE power. For example 10% of electric power. Knowing the efficiency of the ICE engine you can estimate the necessary amount of fuel.
I think there are some points to clarify in your question.
If you think of replacing a gasoline engine by an electric engine working exactly with the same efficiency, then consider that 1 kWh=0,103 gasoline liters. That may work for electric vehicles, not hybrid. Take the Nissan Leaf, for example: its consumption is 21,2 kWh/100 km, equivalent to 2,18 liters of gasoline. However, the efficiency of an electric trail is around 90%, while that of a gas engine is around 33%, so that means that an equivalent gasoline vehicle will waste around 6,0 l/100km of gasoline for the same performance, which is what experience shows for an average hybrid vheicle.
A different matter is the consumption of an Hybrid car compared to a traditional car, because most of the economy of that technology comes from regenerative braking, which an electric vehicle also does. That depends on the type of travel the car has to face. If a car spends its time accelerating to brake in many small urban spaces, the economy can be high, although regenerative braking only regenerates around 3/4 of the energy. If the car spends its time in highways, the economy may be none.
What your asking for has unfortunately no answer. A vehicle is a non linear system : the engine efficiency (its specific fuel consumption to be more precise) varies as its operating point change. For instance, during urban driving conditions, its efficiency is around 20% whereas when driving on highway 30-40% efficiency can be reached.
So if you want to understand the fuel consumption of a conventional vehicle, not only you need investigating the IC engine efficiency (which requires the so cold engine fuel consumption map and its not so simple to obtain it, I can detail if you want) but also the vehicle velocity profile : the torque at the wheel requested by the driver (through the throttle pedal). If you have the data, this is the level of a Master Student.
Now if you want to investigate hybrids, that's even more complex because you're adding another energy source. Hybrids may have different battery sizing, from micro hybrids with very small 48V 10Ah battery or supercaps up to plug in hybrids with 60km range in pure electric mode range. Not to say that some are "simple" system (parallel single shaft) other are more complex (toyota prius).
Once you have fixed everything, that you know all the data, the problem is not solved. One of the main idea of hybrids is to introduce some degree of freedom in the powertrain control so you can optimize a criterion, typically the fuel consumption (and sometimes the emissions). For instance, if you have an electric machine coupled with the IC engine, you can split the driver torque request between the engine and the electric machine.
So to operate the hybrid vehicle you need a control strategy. If you consider an actual hybrid, there is almost no chance to know how the car is controlled (i.e. it is coded within some control unit) and automotive manufacturer generally do not detail this algorithm.
Moreover depending on the initial conditions of BOTH the vehicle (battery initial soc, temperature, etc) AND the internal strategy (it has some internal parameters), you may obtain different fuel & electric energy consumption even if you drive exactly on the same road, with the same speed profile.
So finally you need to express the vehicle energy consumption as a curve of fuel consumption vs. energy consumption over a given velocity profile. There is no way to reduce this to only one fuel and electric consumption. A common approach, although scientifically incorrect, is to tweak the control strategy parameters or vehicle parameter (battery initial state of charge for instance) so that, by chance, at the end of the journey, the battery SOC is equal to the initial SOC. In that case, the fuel consumption can be compared with conventional car fuel consumption. BUT THIS IS A REDUCED ANALYSIS OF HYBRID ENERGY FUEL CONSUMPTION.
So I think there is no simple answer to your question at this stage.
If you want a more precise answer, I suggest providing us with a more detailed question so we can try to reduce the scope to one particular case and, by fixing some parameters, we may find a gross answer to your problem.