I have to recognize that the paragraph of API (mentioned by KP SAHU) is rather confusing. As a detail, the "problem" is that the total pressure has two components: velocity pressure and external pressure (vacuum equivalent)- the last one is not V^2 proportional.


In my opinion, for a velocity V [in specific units], other factors than considered in 5.9.7.1 Note a- giving a Velocity_pressure [in specific units] plus External_Pressure [in specific units] as external pressure, the result should be amended by factor:
1.72/[Velocity_pressure +External_Pressure]
or 36/[[Velocity_pressure +External_Pressure]

Maybe it appears as a complicated procedure; it may be simplified keeping in mind that the formula
H=9.47*t*sqrt[(t/D)^3] in SI units
considers in denominator 1.72 kPa as total pressure.
When we have the total pressure Velocity_pressure +EP we need to consider Velocity_pressure +EP in denominator; this is equivalent to consider in the first phase 1.72 kPa in denominator and to multiply the results with 1.72/[Velocity_pressure +EP] later.