If .357 gives good results than go for it. Your cylinder throats are probably .357 so any bullet you shoot will be .357 by the time it hits your barrel anyways.

The standard lead bullet diameter is .358, but you should use whatever works best in your gun.

A couple of observations - 10 bullets isn't very many. You might not notice any leading until you've shot a couple dozen rounds and once the barrel starts to lead it can get worse quickly or reach a point and stop. Also roofing lead is usually really soft. I'd stick with light 38 special loads. Trying to push soft lead into .357 Mag territories can result in severe leading pretty quickly.