You can't sit in it. It will obscure your accuracy but half the time not the enemy's. What you need to do is pop smoke and THEN back up, so enemy shots passing through it take the reduced accuracy penalty (but also affects your sherman too).
that's exactly what i do, i think it either needs to be bigger, or maybe the fact that i use it so much, it just seems to not be effective (ie i focus more on the times it doesn't work rather than when it does work.) anyone else use these a lot, do they seem effective or no?