Converting a Hireling to a Henchman

In our second session of the campaign the party hired a few hireling (aka red shirts) a torchbearer and some men-at-arms. One of the men-at-arms died. The torchbearer and one of the men-at-arms have pretty much remained anonymous (aka red shirts - "Because my character isn't important enough for a last name, because I'm gonna die five minutes in." - Guy Fleegman Galaxy Quest). But the other Tarl has warmed himself to the party - probably because they call him Carl, not Tarl.

Tarl or Carl?

Or Red-Shirt?

So anyway, if T(C)arl remains a fixture and continues to be treated well, he may become a low-level follower (henchman) rather than a mere hireling. I like to rule-ify aspects of my games. So here is what I'm thinking.

He, and other meaningful hirelings, will receive a number of XP in a session equal to the number of XP received by the PC who received the fewest XP of that session. It is not a share of the XP, denominator (party size) of total XP/party size does not include the hirelings.

I want a good amount of difference in levels between the PCs and the henchman. When the hireling starts to receive XP, I'll note of the level of the PC with the lowest level, I'll call that level Z. Find the number of XP needed from the Character Advancement table in the PHB for a character of 3 levels higher than level Z. When the hireling has received a total XP equal to that number he/she is considered a 1st level follower, picks a class and all that jazz.

We'll see how that goes. Suggestions?