The round bonus is calculated with how many kills you get, how many assist's you get.

It also depends what game type you may be playing, for example HQ, if you capture loads of places you'll get more round points.

im not sure about an actual formula

but i can tell you the better you do the more you get

and like aksoawodnm said its different for game types, like sabotage you get more if you play for longer, you get a lot more if the game ends in overtime.

Higher Level (Rank), More Kills, Less Deaths, Lenght of Mattch, Objectives Captured (HQ, SD Bomb)

Only things I can think off the top of my head

Shots fired, shots hit, kills, whether you died or not, assists, how many times you regained health, distance travelled, headquarters captured, bombs planted, bombs defused, time remaining upon bomb defusal.