-R fully charges its damage after it travels 1500 units (the range of W). This takes a little less than a second.
-R deals 80% of its effect to all enemies in the AoE and calculates the damage on an individual basis. Say you hit a Caitlyn with R at max range and it splashes to the Thresh behind her. Caitlyn will take 450 (+1.0) damage + 35% of her missing health. Thresh will take 360 (+.8) damage +28% of his missing health.