What counts as a partial charge for my rMBP? I just recently decided to switch over to the Mac for the first time by buying the latest Retina MBP (which shipped with Mavericks).

I use it like a commuter where I have it unplugged for a couple of hours but later will plug it in at a desk. I want my battery to last as long as possible and I know I should run the battery calibration check and also have the battery down to 10% once a month. 

However, what about the "partial charges"? What's a good percentage that I can drop down to before plugging it in? I usually start charging when battery is between 50% -60% for now. Is that low enough that it won't hurt the battery in the long term?
"I know I should run the battery calibration check and also have the battery down to 10% once a month."

Apple actually recommends fully charging and then discharging the battery once per month. All modern Macs with batteries self-calibrate and require very little user involvement. There is no best practice as to what level to charge a battery at, none that I have seen that equate to a significant elongation in lifespan.

In short, use your MacBook Pro and don't worry about its battery. Ideally, as long as you are depleting and recharging it, then the battery's health should remain optimal. Running a notebook that is always connected to a power supply is not ideal. Battery cells like it when the electrons flow; unfortunately it is that very motion that invariably leads to a reduction in charge capacity. That is just the nature of the technology. Damned if you do, damned if you don't.

A partial charge is just that: when a battery is recharged without being fully depleted. Apple rates batteries based on full charge cycles. So if you would let yours run down to 50% and then recharge it, twice, then you would have hit one full charge. There is no relevance as to the health of the battery should you recharge it at any given point. Recharge it when it's convenient for you.

The main culprits to poor battery health are heat and inhibiting the movement of electrons (by keeping the notebook plugged in all the time). Avoid those and your's will be just fine.

You can consult Apple's suggestions on the matter.