The main reason to do it in two successive dilutions is that it is (usually) more accurate to say dilute both 25 ml + 75 ml (1:3) and also 1:12.5 than to add simply add 18.5mL + 980.5 mL = 1:53 (999mL total)
This is more accurate than what I posted the first time, remember I rounded up in my first post from 0.0185 to 0.019. So to be more accurate is a 1:53 dilution, or 1 part in 54 total (18.5 + 980.5).
Another issue is the rate at which these solutions deteriorate. A concentrated stock solution will oxidize/deteriorate much slower than a dilute solution in which the pH is not as well buffered and more aeration has taken place (mixing fresh oxygen-rich water with concentrated developer).
But to answer the original question:
1:53 dilution is really close. That would be 18.5mL + 980.5 mL. 999mL total. I assume you have graduated cylinders that can measure an accurate half mL increment?