Q = KiA

Equivalent K = Q / (iA)

K_eq = 90.91cfs / (10' / 1000' * 500' * 10')
K_eq = 1.82 ft/sec

K1 = 90.91cfs / (0.45' / 250' * 500' * 10')
K1 = 10.1 ft/sec

K2 = 90.91cfs / (4.55' / 250' * 500' * 10')
K2 = 1 ft/sec

The addition of a flow observation "breaks" the correlation between K1 and K2, making it possible to determine their absolute, not just their relative values. Remember this if you ever set about calibrating a model by trial-and-error when you have only head observations. Modelers often overlook this problem unless they are using an automated calibration tool that points out correlations.

check:

K_eq = (1000ft) / ((500ft/10.1ft/s)+(500ft/1ft/s))
K_eq = 18.2 ft/sec