First of all, keep in mind that Counter Culture's recommended brew parameters are based on a double boiler espresso machine (La Marzocco Strada), not an HX like yours. In my understanding of La Marzocco-speak, 200°F equates to "medium temperature", 198°F = low, 201°F= medium high, and +203°F = high. I don't have hands-on experience with the Nuova Simonelli Oscar, but I believe it's a Dragon HX design. If so, then medium translates to 4-6 seconds after flashing boiling stops with a 6-10 second rebound.

If you want to confirm this ballpark starting point, read Seeking low tech way to measure brew temperature. Snaking a thermocouple up the portafilter spout is reasonably accurate, as is measuring with a fast-acting thermometer + a Styrofoam cup. That's what I did for a long while before the Scace thermofilter arrived on the scene. The low-tech ways are tedious, but work.

That said, I suggest you not spend too much time with thermocouples, thermometers, and foam cups. Instead, I recommend developing your ability to diagnose via taste. It's not hard, especially if you intentionally choose wide variations. For example, if you flush for time X and rebound time Y one time, then flush 2X and Y the second time, you should taste a distinct change in flavor profile (presumably sour-ish and/or flat). Based on your assessment, halve the difference up/down.

If you haven't done so already, review How I Stopped Worrying and Learned to Love HXs, especially the second entitled "What is the 'right' boiler pressure setting?"