Take the absolute pressure of the fluid (NOT gauge) and subtract it's vapor pressure. Multiply by 2.31 and divide by the specific gravity. That is the NPSH in feet of water that you have available.
Now, this is the NPSH at the point of measurement. If you have a run of pipe going into the pump then you need to subtract the friction loss in that pipe from your pressure gauge to the pump suction.
Technically, you are supposed to correct the pressure gauge to the centerline of the pipe and add in the velocity head term. However, friction loss is an inexact science and you cannot expect to get accurate values (unless you are in laminar flow) to within an inch or two so trying to increase your accuracy with a small pipe and including the few inches of velocity head (since good piping practice states to keep the fluid velocity low) is not worth the trouble. Now, if you have a 6' pipe, then go ahead and correct gauge pressure to the centerline of the pipe.
In other words, USE COMMON SENSE.
I always get a laugh at seeing specs written by "engineers" that require "15643.21 gpm @ 89.012 ft TDH with 38.922 ft TDH." I would have thought that the P.E. process would weed out such "talent."
