Stop Out slotted what turned out to be the winning goal in the first half of extra time, with their goalkeeper pulling off a fantastic save to deny Athletic an equaliser in the last minute of the second, Calvert said.
"It's hard looking back on it but the performance was really good.
"We wanted to make sure we were in the tie at halftime. For us, we had to score two goals and take away their away-goal advantage.
"There were some really big chances to make it 2-nil in the second half but the moments just didn't go our way."
The team had enjoyed a little bit of luck in the regular season but it wasn't on their side on Saturday, Calvert said.
A "massive amount of credit" had to be given to Athletic's opponents.
"Both teams never gave in," he said.
"A couple of days after, for me personally it does still hurt a little bit.
"I'm really proud of our boys. They played a fantastic game on Saturday and they've been fantastic all season."
Saturday's game was Athletic's first loss this season, with two draws the only blemishes on their march to the top of the Federation League.
Calvert said Saturday's result was a bitter pill to swallow but no one could take away the fantastic moments the team had up until that point.
"I said to the boys afterwards that no one can take away their league medals either.
"Every season has ups and downs, there always is and always will be. Our down moment had just been the last week of the season.
"As time goes on, people will remember the 2-1 win over Palmerston North Marist at home, and they will remember getting as far in the Chatham Cup as the club has ever got as far as I've known it."
Athletic made it to the third round of the Chatham Cup before going down 5-0 to Wellington Olympic in June.
Calvert said Athletic were pushing forward as a club.
"It's fantastic to be a part of it, and people in this club deserve success.
"They have been able to see it this year. For me, moving forward, it will be about continuing that success."
