Undiluted coolant in reservoir?

I have a suspicion you're over-worried. Most of us use undiluted coolant because it's very expensive water you're buying with the pre-diluted stuff. When I perform a compete system flush, or a drain and fill, I put in a gallon of straight antifreeze, then a gallon of water, then a half gallon of antifreeze, then a half gallon of water, until the radiator is full. I run the engine long enough to warm up the coolant and have it circulate and mix thoroughly, then I test the freeze point. If it is good to, say, .. Minus 10 degrees, for example, I add straight antifreeze to the reservoir. Similarly, if it's "too good", as in minus 50 degrees, I add straight water to the reservoir. The goal is to get it close to minus 35 degrees.

In your case, I would just drive the vehicle for at least a few days so the coolant in the reservoir gets mixed in, then have your mechanic test the freeze point. If it's good for anywhere between minus 30 to minus 40 degrees, leave it alone.

There's a couple very important reasons why "more is not better" when it comes to antifreeze. The first has to do with how the freeze point is determined. 99 percent of testers use a group of floating balls, or a pointer that sits in the coolant sample. Those simply measure the weight of the coolant compared to the weight of straight water. Antifreeze is alcohol, which is heavier than water. The higher the concentration of antifreeze, the higher the pointer will go, or the more balls will float in the sample. That method is accurate up to a certain point, then things go haywire. Water freezes at 32 degrees. Antifreeze, as I recall, freezes at around minus 10 degrees. It actually turns to slush, not solid like water does. It's when the two are combined that the freeze point goes lower. The best you can get is close to minus 50 degrees. If you hit that, then add more antifreeze, the freeze point goes back up, say to minus 40 degrees, but the testers read the weight, so they incorrectly show it's good for maybe minus 60 degrees. Once you get lower than minus 50 degrees, testers are no longer accurate.

To accurately test the freeze point, you need a tool called a "refractometer". That is a tool where you place a drop of coolant on a sight glass, then look through the lens. There's a scale on that lens to show the freeze points, and the lower half will be dark and the upper half is lighter. That dividing point is where the freeze point is read. That's an expensive tool few of us have or use as it isn't necessary for automotive work.

The second issue with too much antifreeze is it can't hold the same volume of heat, in BTUs, as does water. Water does a good job of absorbing heat from the engine and transporting it to the radiator. Antifreeze does a very poor job of that, so too much antifreeze, by percentage, can lead to an engine running too hot in summer. We do need antifreeze in winter. Since this is a compromise, cooling system parts, including radiators and fans, are sized accordingly.

I should have started out by asking why you needed to add coolant? If you filled the reservoir to the top, coolant is going to overflow the next few times the engine warms up. There's marks on the side of the reservoir to show how full it should be when the engine is cold and when it's warm. Coolant expands a lot as it warms up, and needs a place to go. That's into the reservoir. Once the engine has cooled down for a couple of hours, the level in the reservoir will be rather close to the bottom.

The last thing to be aware of is in recent years there have been some problems with water pumps failing due to minerals in tap water. A number of service bulletins spell out the need to use distilled water in cooling systems. That is one advantage to buying the pre-mixed half antifreeze / half water. That ensures you get the right combination of additives including rust inhibitors and water pump lubricant. The disadvantage is if you need to adjust the freeze point, you still have to buy some straight antifreeze or a bottle of distilled water.

Let me know if I put you at ease or if you have more questions.