How to store iPad when away for extended time

Don't charge it to 100% before shutting it down. Just about anything less than that is ok.

Conversely, don't let it sit completely discharged for months at a time either. Anywhere from 50% to 80% is fine; the lower value allowing for a greater period of time for it to discharge completely.

Excerpted from About iPad and iPhone charge-management feature - Apple Support:

"When they remain at full charge for prolonged periods of time, battery health can be affected."

If you intend to set-aside your iPad for more than a few days, simply charge the iPad and fully shut down - the easiest way from iPad settings:

Settings > General > Shut Down

Occasionally setting aside the iPad with a fully charged battery will do no harm - however, if you do this frequently, you would be best advised to only charge to ~80% prior to shut-down. This measure is intended to reduce the chemical aging that can occur if the iPad is maintained at 100% for extended periods.

If the set-aside period is many months, you should periodically charge the iPad's battery to avoid full discharge. Unless recharged, a fully discharged battery can quickly suffer permanent and irreversible damage through deep discharge.

This Apple Support page provides full advice for maximising and preserving the performance of your device batteries:

Batteries - Maximizing Performance - Apple