So you want to know what fan speed is best?

You can test optimal print fan speed with this tower. You MUST use newest PrusaSlicer and use the fan speed settings in the photo, slice at 0.2mm layer height. I recommend using a large brim around the tower.

The print will start at 0% fan speed at the bottom and end up at 100% fan speed at the top. It increases by 1% every 1mm (and 10% every 10mm) if you use the settings in the photo. Each solid line above the windows are 10mm increments. You can judge the bridging above the windows, any stringing, and the overall surface quality of the print to determine your best fan speed.

I encourage anyone to take my concept here, and remix it into a better design. :)
I'm sure that a better model can be made that will better test the fan, using this same concept.

Thanks

Updated 2/23/2021:
I added the gcode file you can just download it and print for PLA.