One of the rules is, that he has to make direct eye contact. Yet he's able to geass about twenty people at the same time in Episode 1 from Season 2. He even is able to do it later, too. So how can he do this?
On the Code Geass Wikia it says
Lelouch's Geass manifests itself as "the power of absolute obedience," which allows him to plant commands within a person's mind upon direct eye contact.
while yes it does say it works upon direct eye contact it doesn't necessary say that both parties need to make direct eye contact.
i believe that Lelouch's Geass works similar to Superman's Heat Vision
So how focused is Superman's heat vision? Not very, under most circumstances. Most often, it's depicted as a rapidly widening cone, starting at eye-width but growing to several feet across by the time it reaches the opponent, even at close range.
with Lelouch's Geass it expands out and through obstructing this "widening cone" with his Zero helmet he can focus it slightly to one person. from there he should realistically be able to see his target's eyes even if his vision isn't focused on them
My evidence showing Lelouch alone just need to be able to see his target's eyes is in Season 2: Turn 15 - C's World when Lelouch goes to order Charles to kill himself by having the Shinkirō release mirrors and using one mirror bounce his Geass around to target Charles without Charles being able to target him which his Geass too needed eye contact.
this way the Geass expands outwards but the power is reflected to the next mirror and continues reflecting until it reaches Charles as since Lelouch could see Charles eyes in the mirrors then his Geass has to be able to reflect back the opposite way as indicated by how light is reflected1
this would also explain why the United Federation of Nations' counter measure to his Geass was to trap him in an isolated room and display him on a monitor.
my evidence as to Lelouch not needing to focus on the target eyes is when his Geass went out of control while talking to Euphie. it's just my personal interpretation but i do not believe Lelouch was focusing on Euphie's eyes but her face as he was more or less allowing Euphie to win and amend his plans to fit with her own and he was joking about Euphie how he could order her to "kill the Japanese", however he could realistically look into her eyes at any point while looking at her face
While one can argue "Lelouch's power went out of control then so he can no longer focus it" lets remember that his geass was out of control in Season 2: Turn 15 - C's World and when he first gained his power he ordered a group of soldiers to kill themselves and i seem to remember there was times where he would open up one part of his Zero Helmet to use his Geass
1: to put it simply, when you look at a red cube it isn't red it's absorbing every other color of light except red which is being reflected back to our eyes. so if Lelouch can see the purple on Charles's clothes in the mirrors then that means the purple light being reflected from Charles's clothes to the mirrors and then to Lelouch's eyes