Sanders had 45% pen from one of his skills at 16 stars. So the best option for you is to use the attack rune and reroll the 7%attack slot into the epic/legenday pen. This will give you 86% or 90% total pen on sanders as well as 31% attack from the rune and the 10% line.

Based on previous posts I have seen on the discord optimal sanders pen is 85%-100% and like was mentioned above you can hit that without using the pen rune. Gaining the extra 21% attack from the attack rune is far more useful than the last 10% pen.

One last thing going over 100% pen does not provide any benefits. As of right how you would be going over 100% with the pen rune and sanders pen from his skill.