Roundabout advice...

I was taught by an instructor when turning right at the roundabout 'always' to come off on the left hand lane; is this correct? as my stepdad is a lorry driver and he said not to come off on the left hand lane unless its 'clear' which is understandable but my instructor did say if you were coming off on the left hand lane and someone was there/went into you then they would be liable for it. So I'm slightly confused.

http://s33.postimg.org/8blzy6xlb/roundabout.png

Little picture, any advice? thanks.