Nest box capacity

[PheasantsFirst]

I recently rebuild part of my coop and went down to two nest boxes, all then hens(8, not frequent layers) used. I set these up one above the other, but the hens prefer the bottom box. I put a small board in for a ramp to encourage them to use the top box. At how many eggs in each box should I watch for befor I should add a third box, if I even need one. I have been getting around four in a box per day. Hopefully this isn't too confusing

 

[fried green eggs]

They like to lay were other's have already laid. You could put a fake egg or golf ball in the top box to encourage them to start using that one.