You're assuming that your floor is level; this will lead to issues.
What you can do is mark several level spots on the wall which runs parallel with your joists. Since you have a 4' level I recommend putting it on the wall and marking a dot every 2 feet or less so that when you go to mark your next dot you can line up the level on 2 previous dots instead of having to meticulously keep track of the bubble.
Attach string at each spot you marked.
Now run the string to the other wall (the string should be running perpendicular to the joists), use a string line level, and attach the string to the other wall.
This will let you see and measure whether your joists are all level in relation to your string.