As a solo laner you should be shooting for at least 6 cs per minute. My cs per minute in recent top lane games are:
Taric - 6.4
Maokai - 6.4
Taric - 8.3
Taric - 7.3
Rumble - 5.2 (I did terrible this game with a score of 1/8/10)
Tryndamere - 6.5
If you are only getting 3.8 per minute on Zoe, I out cs you in the jungle. You need to work on lane control to not miss waves under tower. Don't go back unless the minion wave is under the enemy tower. This is where warding and map awareness comes in. If you know where the jungler is, you can safely push to the tower before going back. This makes it so by time you get back to lane, the enemy wave will be pushing towards you and you should have only lost maybe 1 or 2 cs.