The 52-year-old Australian, who is winless on the senior tour, leads with a seven-under 133 total.
"You can't fake it around here. You've got to hit it solid," Percy said on Friday.
"The par 3s are hard, everything's hard. You've got to hit it really good. You can't just slap it everywhere and shoot a 68."
Percy and his caddie have done the maths to decide what it should take for him to post his maiden win.
"We worked out most of the time if you get to 12 under, you'll win," Percy said. "So I didn't care about my score, I'm just trying to get to 12 under. That's three-under a day and I'm one shot ahead of that.
"Really, you know, I'm still five behind what I'm trying to do. I need five more under from here on in and if someone beats me, well, that's just too good."
First-round leader Jerry Kelly is tied for second with Zach Johnson. Kelly shot 69, and Johnson had his second straight 67.
"A few of these holes out here really make you think," Johnson said. "Seems like the harder the hole, the more difficulty I seem to execute even better. That was encouraging."
The 50-year-old Johnson has two victories in eight senior starts this season. He skipped the US Senior Open last week to play in the PGA Tour's John Deere Classic, where he tied for ninth.
The 59-year-old Kelly won the event in 2020 and 2022.
Boo Weekley is five under after a 67.
"I reckon I'm just a little more mature at this golf course," Weekley said. "I know, that's kind of hard to believe for me to say that word. I can't spell it."
Akron native Ryan Armour, who turned 50 in February, had his second 68 to join 2019 champion Retief Goosen (67), Darren Clarke (70) and Tag Ridings (66) at four under.
Charles Schwab Cup leader Stewart Cink is three under after a 68. He has four wins in 11 starts this season.
Australians Stuart Appleby (68-73) and Greg Chalmers (69-72) are tied 26th at one over, while Rod Pampling (71-71) is one behind them.
This is the last time the tournament will be held at Firestone Country Club, ending a relationship with the PGA Tour that dates to the Rubber City Open in 1954.
