Not sure that it should really matter.
I think you should just focus on obtaining conversions efficiently and getting a volume with which you can deliver a good outcome (that could be shipping product quickly or completing service quickly, not overwhelming your resources etc.)
Impression share just shows you how much ramp you have left in your specific market (vertical + geo).
Whenever I’ve come across people with impression share goals (i.e. I want to blanket my competitors), it always wrecks the balance of efficiency and service I mentioned above.
Most markets are big enough to sustain you and your 15 competitors. Growing your slice of the pie and forcing 3 of them to close down is a marathon, not a race.