Sean Lineen won 29 caps, making his debut against Wales in 1988, going on to enjoy a distinguished Scotland career, including the 1990 Grand Slam.
In today's Scotland on Sunday, Sean talks about the prospects of Calvin following his footsteps into international rugby.
"Tactically and technically he has become better since I first saw him six years ago," says the Glasgow's backs coach. "Calvin's one of the best passers of a rugby ball I've ever seen. He's been the catalyst for the turnaround at Glasgow - he's keeping Dan out of the team on form, and that takes some doing.
"If he keeps performing consistently for Glasgow, then it will make it impossible for the selectors to ignore him any longer.
"I know I was in the right place at the right time - there was a gap at centre for Scotland because I wasn't a great footballer," says Lineen, who brought Howarth to Scotland through a contact in the Auckland club rugby scene.
"Although he's probably third choice, behind Parks and Gordon Ross (of Leeds Tykes), Calvin's the form No.10, and like me, he might just be in the right place at the right time."
"Calvin makes other players around him look good; he puts people in space, and can terrorise opposition defences. He reminds me of Wally Lewis, the rugby league exponent.
"He's taken a while to adapt to professional rugby - defences close you down quicker. But he's grown up, learned to be more patient and how to control a game better, to get a better balance.
"I used to get really frustrated watching Calvin: he's a phenomenal talent, and when he gets the top three inches working properly he plays with real confidence."
Calvin also thinks his coach has influenced his, and Glasgow's, improvement in fortunes.
"Sean's made us more threatening and explosive, moulded Glasgow into the best back-line in Scottish rugby," says Calvin. "His personality and style suits the way I want to play. I've benefited tremendously from having someone around like him, who knows me so well."
"I'm not focused on Scotland. I want to establish myself in the Glasgow team," Calvin confirms. "I don't think the Scotland call's going to come. Matt Williams' has picked a young team, and will persist with them for the next World Cup.
"It would be a major bonus but, personally, I'm not waiting by the phone for it to ring. I went through the stage where I had a great opportunity to force my way in, but didn't grasp it.
"If it's anyone's fault, it's mine. I put myself under too much pressure - I lived and breathed rugby 24/7, and got too anxious."
"If a Scotland call-up comes, of course I'll grab it with both hands. I'd like to break into the Scotland team, but I need to be performing week in and week out for Glasgow - starting on Friday night against Toulouse."
You can read the Scotland on Sunday article here