Generally, I would aim for lash fans that are uniform and symmetrical with each individual lash of that fan facing in the same direction. Creating fans where the lashes split symmetrically but all face upwards will provide lift to your set. However, looking back on my earlier sets I can see that my fans didn't necessarily have that uniformity in direction. Depending on what pick up technique I used, I could see that some fans tended to be more open and angle outwards from the centre rather than all facing upwards. I call these 'Open V Fans' as opposed to 'Deep V Fans' that I usually create.
Open Vs can give more emphasis to your set horizontally, lengthening the look of your set so would be ideal for clients with small eyes.
To create uniformity and lift
Overlap more for a more feathered look