Which grip will allow for the highest number of reps

Hi

For the context - i'm currently training for a firefighter academy physical fitness test.

The test requires pull-ups, where 26 reps gives the maximum points, and this is my target.

There's half a year left until the test and i'm currently at 13 reps in both underhand and overhand grip pull ups.

I think I should start doing pull-ups with just one grip, and my question is - which of them will make it easier to achieve the goal?

I do calisthenics only twice a week, because on the other days I have to train running. I also don't know if I should be doing any biceps and triceps isolation exercises. I do pull-ups, dips and push-ups but i don't know if it targets my arms enough.

I would be grateful for your advice