Johnstone said to achieve such a victory win in dramatic circumstances - Melbourne trailed by 27 points during the second term - will be crucial for next week's challenge against the loser of the Adelaide and Fremantle qualifying final, which is being played at AAMI Stadium on Saturday.
"To come away with a win like that was fantastic … especially a hard-fought win. To be down by 27 points … I think they're the games that you really get your confidence from," Johnstone told afl.com.au.
"We could've been up, from the start and from the first bounce, up by four or five goals and stayed that way for the whole game and you come off and you're pretty confident … but we really had to fight hard."
"Come next week, if we're in a tough situation, we can think back to St Kilda and have that belief that we were in a tough situation, so why can't we do it again? A lot of boys will benefit from that."
As for next week's opponent, Johnstone said either the Crows or Dockers would represent a tough contest.
"Adelaide is playing good footy and so is Fremantle - as everybody knows - they're the in-form team, so it's going to be hard, no matter whoever we play, so as long as we don't get too far ahead of ourselves and think our luck's completely changed - we've got to stick to what we've been doing and just stick to it," Johnstone said.
"Hopefully, we can come away with another win."
