WALTHAM — There’s a reason Ray Allen is regarded as one of the best pure shooters in NBA history: Practice, and lots of it.

Allen’s regiment for shooting nearly matches the same for his conditioning. Allen will spend 45 minutes or longer following practice, just practicing catching and shooting from long range. He’ll usually shoot from both baselines, both wings and from the top of the key. (In the video, he is taking passes from Celtics assistant coach Tyronn Lue.)

Allen enters Tuesday night’s game vs. Cleveland needing just 24 3-pointers to pass Reggie Miller (2,560) for first on the all-time list.

He is on pace for 180 this season, which would match his total in 2008, his first season in Boston. Now in his 15th season, Allen has led the NBA in 3-pointers three times in his career, each time passing the 200-mark for a season.

Allen set the all-time single-season NBA record for 3′s in a season when he drained 269 of them in 2005-06 with Seattle, two more than Dennis Scott with the Magic 10 years earlier.

This year, Allen is fifth in both 3-pointers made at 93 through 43 games and 3-point percentage at .452.

After undergoing knee surgery in May, Kevin Garnett returned to the Boston Celtics training facility in Waltham on Monday,  Marc Spears of Yahoo! Sports reported. While the organization hopes he will be cleared to participate in full-court scrimmages for the start of training camp, they are proceeding with caution.

According to Spears, Garnett is currently limited to court drills but could be cleared for contract drills as early as a week before training camp begins. The Celtics main concern, though, is not Opening Night but rather the postseason.

“We’re looking for him to be there at the end of the season, not just the start of the season,” Ainge told Yahoo!. “We’re getting him ready for training camp and looking forward to him being healthy in the playoffs. He looks good. Really good.”

Garnett has not played since March 25 when he scored four points in 17 minutes during a loss to the Orlando Magic. The Celtics forward, who has not averaged less than 31 minutes per game since his rookie year, has not played over 30 minutes since February 12.

Ray Allen, Paul Pierce, and Rasheed Wallace have also been working out at the facility.

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In Celtics backcourt news, Ainge told Yahoo! he has not decided whether or not the Cs will offer Rajon Rondo a contract extension before the season. In the meantime, the Celtics are considering the addition of a third point guard. Free agents Tyronn Lue, Dan Dickau, and Mike Taylor worked out at the team’s training facility on Thursday.