Pure coolant to the reservoir or?

Okay, so I was changing my valve cover and needed to remove a coolant hose and when I was putting everything back together and was going to refill the coolant I noticed some leaves entered the coolant tank when I took the cap off.

I then removed the tank and rinsed the leaves out with distilled water and vacuumed the connecting hoses just in case some debris fell into the hoses. I then reassembled everything and decided to flush the coolant.

While it was draining I added a gallon of distilled water to flush more from the system.

Then I bled the system and ran it for a bit.

I ordered a refractometer to get the coolant concentration I desire.

Am I able to add pure coolant to the reservoir or do I need to add premix?

When I drained about half remained in the engine.