The most important question in convection heat transfer analysis is the calculation of the "heat transfer coefficient".
Once you know the Nusselt number you can easily calculate the heat transfer coefficient using
h = heat transfer coefficient (W/m^2.K)
L = characteristic length (m)
K = thermal conductivity of the fluid (W/m.K)
But the calculation of the Nusselt number is not as straight-forward as it seems. There are multiple empirical, analytical, and experimental methods to calculate the Nusselt number and it always comes with errors. Hence, it is important to use appropriate formula for Nusselt number.