Kalista Rend Damage

Can anyone confirm the formula for Kalista's Rend in relation to AD and Rank? I'm trying to math out stat priority for Kalista's Rend to get max damage/effectiveness between Armor Pen, Attack Speed, and AD. For example I want to know what stat would lead to a harder hitting rend if you only had 3 seconds to attack your enemy before rending - would more spears from attack speed be better? Or less spears and more AD? or would armor pen lead to a harder hitting rend in the same time?

After testing 1, 2, and 3 spear Rends, at ranks 1 and 2, between a 30 AD range in increments of 10 AD, I found it seems to be roughly: +1.6 damage per spear per 10 bonus AD +2.2 damage per spear per rank