An iPad battery is rated to maintain at least 80% of its initial capacity after 1000 full charge/discharge cycles. Simple math therefore suggests that your iPad battery health can be expected to fall by 1% for every 50 full charge cycles.
A full charge cycle is any combination of charge that together sums to 100% (e.g. 40% + 20% + 20% + 10% = 100 = 1 full charge cycle).
As such, while screen brightness and CPU/GPU loading will influence how quickly the battery will be depleted while the iPad is running on battery power alone, it is the number of charge cycles that is the principal determining factor for a reduction in battery health.
While limiting charging to 80% of full capacity will usefully reduce chemical aging that will influence overall battery longevity, you will be recharging more often. This strategy will not reduce the cumulative effect of charge cycles - as a full cycle is any combination of partial charging that together sums to 100% - even when limiting the battery charge to 80%.