owillis --
The selection of those flow numbers isn't completely arbitrary - notice how the flow range begins at 400 gpm and ends at 1200 gpm - that corresponds to the table of single/two(parallel) pumps. Typically you want an EVEN distribution of values so you can draw a smooth curve - so in this sense the selection isn't arbitrary. However, you could have picked a flow of 988 gpm to calculate and plot the corresponding TDH instead of 1,000 gpm - you would still be generating the same curve since those points would fall along the same curve. It's just not as easy to plot 988 gpm (at least by hand).
Big picture - think SIMPLE and EASY.
JR