I have noticed on my RZ that if using an RZ lens, when the bellows are fully collapsed, I am focused on infinity. When using an RB lens, I need to advance the focus a couple of mm's to get the lens to focus on infinity (i.e., then the bellows are fully collapsed, I am focused on nothing). However, what you see is what you get and as such, you should always focus on what you need to be focused. So, you might be right in that an RB lens needs a couple of mm adjustment but do not make this adjustment; look at the glass and when it is in focus there, it will be in focus on the negative.