I will be flushing out the old red coolant out of my 2000 GS and adding Toyota RED concentrate.
I plan on using a couple of gallons of distilled water to get most of the old red out of the heater core.
What is the best process to ensure and equal 50/50 mix. I understand that some distilled water will remain in the heater core.

 

If you really want to get a good mix you can take an empty distilled water jug and mix coolant and distilled water in the jug or in some other empty container

 

Or, you could just do a drain and refill with 50/50 and not worry about it. I was shocked when my 150k miles coolant flush came up. LOL. As per the service manual instructions, Open the valve and drain the coolant. Refill with fresh coolant. That's it. If your cooling system has already been well maintained, you will be fine. If it makes you feel better, do another change in a week or two of driving. Any issues with mixture will be settled.

Paul