How [much] to bill out a Junior developer

I'm an independent senior developer, and looking to slowly grow a small team of devs. A common theme I've run across is people want to mix a senior, and junior dev to produce a lower "mixed" rate for a team.

1. What's a fair rate to bill out a Junior Dev at. My focus is on devs who have little to no real-world experience, and a 2 year degree. (Recruiting from a local, technical college).

My thought is that with some of the very "green" junior devs, fresh out of college, I would bill them out at (common rate) / 2.

(i.e. $125 / 2 = ~$62.50).

With 6mo-1.5yr experience, and/or depending on the individual, I would nudge the formula to be (common-rate) / 1.5.

i.e. ~$85

Eventually, they would fall into the mid-senior category, and demand the common rate.

Just curious to hear thoughts on this approach, and what you've encountered in the industry.

Location: Mid-west / Wisconsin Primary technology: Web / .NET

Note: I know rates are very relative, but I'd be curious on the formulaic approach.