Ok, here we go! For context, the Reynolds' number of a U2 is roughly 27 million, by my calculations, and if my understanding is correct a smaller Reynolds number indicates more drag.
To calculate the Reynolds number of an ISS-substitute we need estimates of the altitude we want it to be (h), and the length of its journey (d). Technically we should work in metres, but it's a pain enough to use this so I kept h in km as that never actually enters into it; it cancels in finding L, and is only used in the main equation to approximate its corresponding density or viscosity.
If you want to derive this yourself:
https://aviation.stackexchange.com/questions/24641/what-is-the-relation-between-an-airplanes-altitude-and-the-drag-it-is-experienc/24644I used that, plus polynomial interpolation on what data I could gather. It should be more accurate from 0-85km, up to the thermosphere, but data on viscosity is lacking there beyond a paper that states in the abstract kinematic viscosity tends to zero, and so Reynolds' number would become infinite and drag as such minimal. L comes from proportions, something to simulate what the ISS is meant to be, V is just a basic calculation of velocity.
Looks awful, but is generally pretty simple if tedious to apply. Can be extended above 85km, but is likely to become unreliable the further you get.
The denominator is the most unreliable part. The numerator used more points to be accurate up to 200km. If I find more data, I'll update this.
This is probably most useful for those that have the software to program something that does all the work for you if you input the values. I used to, but not on this computer. Calculating directly is possible too, and definitely easier, if you have the data.
For example, the ISS mentioned as a lower bound, travelling at 5833m/s, at an altitude of 80km, has Reynolds number of almost two hundred thousand.
For comparison, the NASA X-43 rocket plane, which flew at a more defined altitude than the Falcon and is otherwise the fastest aerial vehicle I could find record of, had one of 17 million, meaning significantly less drag. If you want to run the numbers on other planes, please do and add them so we can get an idea of what kind of number makes for a feasible flight. Direct is probably fastest without some snappy way to get this approximation more wieldy, to be honest, making that a waste of time but hey, it was fun.