[Tom Hill]

Response for Angelo,
I'm flattered, but IMHO still no expert. Although the units of viscosity are omitted from your post, I think that Mr. Gaudich's calcs look reasonable and your hydraulics scenario is in the critical flow regime between laminar and transition to turbulent. From a programming standpoint, you might consider determining the friction factor in the critical flow regime by use of a cubic spline curve fit. It is fairly easy to fit a cubic spline through four data points using Legendre Polynomial of third order. The four data points can be defined as follows: (RE1, ff1), (RE2, ff2), (RE3, ff3), (RE4, ff4) where RE values represent four Reynolds numbers at values of say 2000, 2400, 3800, and 4000. The ff values represent the friction factor values corresponding to those Reynolds Numbers. Use 64/RE1 to determine ff1 and assign the same value to ff2 as well. Use Colebrook White Equation to determine ff4 and assign the same value to ff3. This will adequately define fours points through which a cubic polynomial will fit. I realize that this method does NOT provide for continuity of the slope of the ff curve at RE=2000 and RE=4000, but it's close enough. A mathematical purist might choose the four knowns to solve as the values of ff at RE=2000 and RE=4000, and use the first derivative of FF=64/RE at RE=2000 as the third equation, and first derivative of the Colebrook-White Equation at RE=4000 as the fourth equation to determine the applicable third order polynomial. This is far too messy. Besides that, the derivative of the Colebrook-White equation woud would likely have to be solve numerically rather than analytically all of which is a severe waste of time.
Tom