Math Help: D&D 5E Barbarian Rage

I am trying to come up with a formula to represent the Rage Damage for the 5E Barb, but I'm not sure there is a simple way to do this. The simple one I've come up with is floor(Proficiency/2)+1 but that seems to break at 16th level, and only works for a pure barbarian who doesn't multi class. The other one I've tried is ceil(BarbLevel/8)+1 but that also breaks at 16th level. Is there a method that will work? It's easy enough just to use an attribute on the character for rage damage and leave it at that, but my brain still wants to know if there is a mathematical solution. For reference, Rage damage starts at +2 for 1st level, goes to +3 at 9th level, and to +4 at 16th level. Proficiency is +2 for 1st, +3 for 5th, +4 for 9th, +5 for 13th, and +6 for 17th.

I think that you would need to add in what is basically an "IF" function or two in order to work around it breaking. I know how I would do it theoretically with the 3.5 character sheet but I'm really not too hot on the 5E stuff. However, I can take a closer look during the week and see if I can give you anything more concrete.

Perhaps not the most Elegant Macro but this will work for Calculating your Rage damage: [[2+{floor((@{barbarian_level})/9),1}kl1+{floor((@{barbarian_level})/16),1}kl1]] RAGE ON!!

I used this one it's about the same as the above. [[(floor((@{selected|barbarian_level})/9)+floor((@{selected|barbarian_level})/16)-floor((@{selected|barbarian_level})/18)+2)]] There is always more than one way to do things in with this system.

Thanks for the answers folks!