Langfield dived to his left to push away the ball and earn the Dons a 1-1 draw after Jamie Murphy and then Brian McLean headed on Jim O’Brien’s inswinging cross.
Calderwood said: “It was a wonderful save from Jamie Langfield in the last minute.
“It could be a massive save and a massive point for us.
“I think it’s a point gained and definitely not two lost. It’s helped our first aim, which is the top six, and hopefully we can finish it off next week.”
Motherwell manager Mark McGhee was disappointed his side had not built on their position of power in the game.
He said: “At 1-0, I always felt we needed another goal.
“I think we were the better of the two teams.
“We made bad decisions at times when we had the opportunity to create clear-cut chances.
“And then towards the end when we went close, it didn’t quite go in for us. Their keeper has made a good save, but we are reasonably satisfied.”