Sorry, it is unclear as I wrote it in that post.
The rule applies to the 1:2 ratio, so you want to set the grinder so that you get for example 18g - > 36g in 36 seconds. That would be a starting point.
The 51ish grams output in 51 seconds would ask for a 25ish grams input (there's a 25g vst basket on the market).
What I do is that first I dial in the grinder for the above mentioned rule and then when making espresso, I cut the shot before I reach the 1:2 ratio. So, it is not a real ristretto but a shortened shot. From 15g input I want to get around 20g, from 18g I usually