[itsjustascratch21]

If your topping up with that, you'll be fine. Tap water is fine too, adding coolant for a top up is overkill.

If you're performing a drain and fill 80/20 is probably okay depending on how much coolant you have on hand. I wouldn't worry about it too much water under pressure has good heat capacitance.
The heater core hoses and o-rings take a beating in the summer if you run the air conditioning in traffic.