I'm guessing these work for diesel type coolants too, ie Zerex G-05?
https://www.walmart.com/ip/TESTER-COOLANT-PRESTONE/16817506
https://www.walmart.com/ip/TESTER-COOLANT-PRESTONE/16817506
Those work to tell you what level of protection you have for boiling or freezing.
Maybe I oversimplify, but I always have an empty coolant jug or 3 floating around, and most seem to have a measurement on the side. Pour in one of the two, up to the half mark, fill with the other to the full mark. Now, I have 2 halves left, either pour them together and now I have 2 gallons of 50/50ish, or add to both, now you have 3 gallons......
This is a 30 year old diesel engine, not going to the moon.....
I think that you'd be almost entirely antifreeze if you do this. If you have an idea of how much you drained out the very first time and you drain it the exact same way after the flush, you'll have a pretty good idea of how much water is left in your cooling system. That way you can add enough straight antifreeze to make the mixture close to 50/50. Then top off with 50/50 mix. If you have two gallons of water left, then put in two gallons of antifreeze. It's really not too hard to do. By the way, I've literally replaced hundreds of these engines and the most coolant that I've ever put back in was maybe 6 1/2 gallons. That's in pickups and I believe vans (really can't remember for sure). That's with a new, dry engine and blowing out the heater core.I suppose just putting in 5 gallons of concentrate first, then only ever topping off with water as it slowly burps itself over several days of driving would work too.
What's the total capacity? About 7 gallons or so? After you flush put in 3.5 gallons of pure coolant then top it off with water.
I think that you'd be almost entirely antifreeze if you do this. If you have an idea of how much you drained out the very first time and you drain it the exact same way after the flush, you'll have a pretty good idea of how much water is left in your cooling system. That way you can add enough straight antifreeze to make the mixture close to 50/50. Then top off with 50/50 mix. If you have two gallons of water left, then put in two gallons of antifreeze. It's really not too hard to do. By the way, I've literally replaced hundreds of these engines and the most coolant that I've ever put back in was maybe 6 1/2 gallons. That's in pickups and I believe vans (really can't remember for sure). That's with a new, dry engine and blowing out the heater core.
When I flushed mine, i pulled the water jacket drain plugs (hint, replace the plugs with petcock valves, makes future drains quick!) from the block as well as pulled the lower radiator hose and drained the radiator. The whole system seemed pretty empty at that point. When I went to refill it I premixed all my coolant to 50/50 and it took close to the specified amount indicating there wasn't much more than a few cups of water left in the system.Except several gallons of water is going to be in there after backflushing. I suppose just putting in 5 gallons of concentrate first, then only ever topping off with water as it slowly burps itself over several days of driving would work too.
Since this is titled as a 1985 model but was probably sold as an incomplete vehicle, the engine may be old enough that it has the awful plugs that took a 1/4" male square socket instead of the nice brass ones. Those others like to sieze (rust?) in place and don't like to come out without a drill and an easy out. Just a warning for the future.