Charging MacBook USB-C with power supply.

[1337GameDev]

I am also curious if there's anything special about this.

Currently, I believe to guess power usage you can inline an ammeter.... but im not sure about current limiting.

 

[Holes Flow]

The charging circuitry is inside the laptop-that is, the current and voltage regulators.
You can charge either on or off.
For on, you just need to know the 'normal' current draw (without a batter in). That is your target. [1]
LiPo (and other) batteries charge by hitting the device with full current, and that current trails off as the battery's voltage approaches what it is rated at. You likely have a few cells at 3.7/4.2V that are series/parallel to create the voltage it says on the outside. This voltage should not be confused with your [external] charger's output voltage.
Plug the charger in, and watch the PD negotiation at the magsafe area. Note which voltage it "stabilizes' at. Remember that the Type C standard is a MAX of 100W (IE, or I*E).
Knowing this, set your PS max current to comply with that voltage and W limit.
You might be surprised to find out the max current for each of the several voltages the PD spec can produce. Check out figure 3 here for a valuable reference, along with the surrounding explanative text.
Your laptop will only take what it needs from your PS, and you can safely leave the bench PS on 24/7 without impacting the laptop.
The only 'trick' to charging is knowing when done. And done means that the voltage is at max (for your series/parallel battery array), and that the current draw is damn near zero. The laptop guts are doing both for you, but you can watch with the PS by looking at current draw (when either on or off, discussed above at [1]).

1. You also have the option of using PD "trigger" cards to set a maximum, or even fixed, voltage supplied at the USB-C connector. You can then time or graph the charge times based on supplied voltage, learning a bit about your battery/power system.
2. This is yet another justification for why I bought a Digital Load! (and they said I was crazy...)

HTH