First off: What is the minimum amount of coolant you can/should run in a car in TX?

I use Toyota Red coolant so I can run extended drain intervals. It's been 2 years and almost 30k, so I did several drain and fills (essentially a flush) this evening. I did all this in my driveway, which has a fairly steep incline so that I could let the water run off during the drain and fills. The last time I flushed the system was at a friends house on a flat driveway, so I just did everything the same.

After 3 drain and fills the water came out clear, so I was ready to fill it back up. I had premixed a gallon of 50/50, so I poured it into the overflow resevoir first and then poured the rest into the radiator. Here's where I believe my problem stems from; since I was working on an incline this time I don't believe as much drained out of the engine as last time. I was only able to fill up the overflow resevoir to the Max line and pour the rest of the 50/50 mixture into the radiator before it started overflowing.

That means that 50% of a gallon of coolant is inside my cooling system. 2/9 quarts = 22%. I don't really need 50/50, but I thought I should run at least 35% or so. I never even considered having less water drain out due to the incline Depending on suggestions from here I can always drain out a small amount of the mixture and put in pure coolant to bring the percentage up. The problem is that all this needs to happen before tomorrow when I go to work.

The last thing I need are overheating problems.
-hype