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Your install with 90 deg elbows should be fine. If you want to get technical, you can google online and find a derating table that lists the feet equivalent of various components. You then add those feet equiv to the total run length, giving you the effective length. I've seen them in install instructions for range hoods. As an example, a hood might be spec'd to work with a max duct length of 30 ft. You have 10 ft of actual duct, two 90 deg elbows (5 ft from chart each), a
45 deg elbow (2.5 ft). That's an effective length of 22.5 ft.I'd look at the install instructions for your fan to start. But for a small bath situated by an outside wall, the duct plus two 90's should be fine. The 5ft equiv for 90 deg elbows is I believe correct for larger round ducts for hoods. In new construction, I've seen them vented from a bathroom on the front side of a house all the way across to the back side of the house. Yours has to be a lot better than that.