The way I decide who to cull-is by the activity and overall appearance. And this can change based on goals and needs regarding color and fin type, but I always cull the unhealthy, deformed...etc.....
I start with any that look to have buoyancy, clamp fins, deformities. One that seem to be less active or showing abnormal behaviors.
Generally, in the first week or so-I will cull half the fry-then over the next 6 weeks-I may cull by half again. Usually by 6 weeks I will have culled down to about 30-50 fry. Then after 6 weeks I may cull that number by half...it depends on what my goal is for that spawn and/or how many I need to meet an order. But I like to cull hard since I have so many spawns-Plus, in my opinion...quality is more important than quantity.....The Bettas I rear-I want them to be healthy so that the new owner won't have any issues related to breeding/rearing...Its out of my hands once they leave my fish room-but at least I know I am sending out good stock to start with.