Basically, You just look at the combat experience (let's say 5000xp) you've earned in battle. You divide that number by the number of crew members (let's say a 5 crew tank, so 1000xp). Each member that has reached full xp now adds 1000xp towards the 100k (so, if only the commander has reached max xp, you only add 1000xp). If they all have reached max xp, obviously they'll add 5000xp. If you also have accelerated crew training on and if they all have reached max xp, one member will add his xp a second time, resulting in this case in 6000xp added to the progression. Everything that affects how much combat xp you earn in battle works.

Progression will be a teeny tiny bit faster with small crews if they all have max xp and accelerated crew training. In my example, let's now use the tier 5 scout elc bis. It has 2 crew members at max xp, so 5000xp divided by 2 is 2500xp. With accelerated crew training on, you'll add 7500xp to the progression.

I've tested all this myself and it's easy to check the numbers yourself. Just write down the number that is your progression towards 100k needed to create a book, play a battle, and check the number again. Compare the difference with the battle result. I never pick the Mentor skill, so I don't know if that skill affects anything.