So, funny question. Let's say 100 people join a game, they all do one "round" of runecrafting and 99 players leave the game. Does it essentially autocomplete?
My understanding is that it'll recalculate its existing % and the charge needed to complete the game dynamically, rather than just ending the game, so you'll probably need to do about the same amount of work yourself (if not a little more since the 20% penalty for players beyond the 20th is no longer a factor).
Also if 100+ people specifically join, the scaling shouldn't decrease below that 100 - so strap in for a long game if you're trying this!