Here are the steps I would follow if I were you, and you'll only be off on a single batch:
Before you brew, get a notepad and a pen, and keep it handy. Then use Beersmith, or one of the other two programs to calculate your target strike water temp.
Boil up a tea kettle full of water, and keep a couple ice cubes handy in case you are way off, and then bring your strike water up to temp using your thermometer.
Dough in your grains and see how close you got. Now before you go adjusting the temp, right down in your notebook the temp Beersmith told you to hit, the temp you hit, and the mash temp you reached.
Now add some boiling water if needed, or if you're a couple degrees too high, just stir the mash with the lid of your cooler open for a few minutes until it cools off a touch.
Now you know exactly how your brewing system reacts to the strike water infusion. If the beersmith calc came up a couple degrees too cool, you know how to adjust. If it was too hot, you know how to adjust. Just take EXCELLENT notes the first time through, and you'll only miss your mash temp once.
Trust me on this, I didn't learn the first 2-3 times I went All-grain, so it wasn't until my 4-5th time I was confident i would hit my mash temp.