All sorts of options.
First, wind chill is one of my pet peeves: We all know it feels colder with wind. Most of the public is misinformed, off quoting all kinds of nonsense about how cold they heard it was, etc. mixing numbers because the TV stations use wind chill and "feels like" to create something that sounds more exciting. It sells TV watching better. I'd be happy if they were prohibited from quoting wind chill numbers and EVERYONE would be better off.
In your case, for the innards of a tractor cooling system, wind chill is irrelevant and has no meaning. No wind inside your radiator. But that said, you guys in central Ill. sometimes (not a lot) get zero and subzero real thermometer temperatures, right? If you can rely on it being in the garage overnight all winter you probably do not need to do anything -- that is unless you think it is going below 10 deg in your garage. Of course Peter is right in suggesting the overall standard recommendation for the entire country -- flush and refill with 50/50 -- which gives you a 34 below freezing point. By the way, once the tractor is warmed up some there is no danger of the coolant freezing, though it possibly could in VERY extreme cold in the radiator because the thermostat might be closed for a while not circulating into the radiator.
I see nothing in the world wrong with removing a quart or so of your coolant and refilling with 100% antifreeze. That will lower your freezing point several degrees and you'd be fine for the winter because of your situation using the garage. You may even have a little heat in the garage in the most severe weather? Obviously draining a lot more and replacing with 100% antifreeze will give you more margin until you reach the 50/50. Watch out as they are selling 50/50 mix in jugs these days.
Probably the practical question is "When was the antifreeze last changed and should it be flushed and refilled anyway?" If it is old (and it probably is because no one is going to flush it and fill it to only have a +10 freezing point) then Peter is right and the best answer is just have it over with -- flush and refill.
But this whole thought process raises what MAY be a more important question: Why did prior owners have the cooling system apart, put in a bunch of water and then only enough antifreeze to give +10 freezing point? I would press that question hard with the previous owner first.