We are not talking about trial games we are talking about competitive league games and 18 is way too short most teams in NPL other than Jets Youth think so , we have just recently been relegated from their and had the league been over a slightly longer format I question if Valo would have gone down but fair enough after 18 games we were bottom and the table does not lie.
Also cup games ? The state cup pretty much no newfm team ever play more than 1 round due to the stupid way it is drawn so again kind of irrelivant to this conversation.
And as you say yourself + finals only for those that make it so vast majority of teams in both leagues again makes no difference so yep again 18 game comp does not cut it even using your calculation
Based on a 10 teams newfm comp in total (not including state cup) newfm
Only 2 teams will make it to Grand final and so they have maximum 21 game comp (18 league games & 3 finals games)
2 teams (losing semi finalists) will have a 20 game season
the other 6 will remain on 18 game competitive season.