Meh:
http://www.xflr5.tech/docs/Part%20IV:%20Limitations.pdfIf you enjoy experimenting with it, and learning as you go, then great.
It's not a real answer for any calculation you give it, though.
It's most accurate if your wing is a perfectly flat plane. If you have an airfoil, it's only an approximation. The more thickness and camber, the poorer the approximation.
Yes, that "5.436" would give me pause, too.
The advantages to perfect airfoil selection will be very difficult to measure. Won't be obvious unless you build two complete sets of blades and measure performance from them both. With fluctuating wind, the difference will vanish in the scatter of the data you collect.
Just get the angle of attack right, and a smooth surface, a fair rounding to the leading edge, and a sharp trailing edge.
My HAWT blades are pulling in a CP of about 50% +/- 10% with an approximate NACA airfoil. I really don't care about the remaining 9% enough to make another set with another airfoil on the speculation that it will improve anything within my ability to measure.