Trainer John Luttrell revealed the plan after the five-year-old scored a narrow win over the odds-on favourite The Risk Factor that was backed as if unbeatable, firming from $2.45 to $1.35 before easing out to start at $1.50 when good money came for Luttrell's mare.
It was a good win, aided by an 8.5kg weight advantage for Luttrell's mare, courtesy of a four-kilogram claim for apprentice Johnstone, and that proved to be a telling factor at the business end of the race.
Encosta Fiorente led with rider Brendon Mccoull content to allow The Risk Factor to settle second and that's how they raced to the 500m peg.
McCoull urged his charge forward to get on terms with Encosta Fiorente and just when it looked as if he might have the edge, Johnstone called on her mare to dig deep and she responded magnificently.
The winning margin was narrow, but it convinced Luttrell his mare has what it takes to be very competitive over the Hobart and Launceston Cups journey (2400m).
"She can go to a similar graded race in Launceston in a fortnight or I can give her a short break and aim her at one of the minor cups, probably the Brighton Cup and if she won that we'd probably progress to the Hobart Cup," Luttrell said.
It was the second leg of a winning double for Johnstone who delivered one of the best rides of the meeting to secure victory aboard Gee Gees Jennia in a 0-62 handicap over 1420m.
The mare drew the outside barrier, but Johnstone had her three-back on the rails after travelling 300 metres and she persevered for rails runs in the home straight and that proved to be the winning move.
Johnstone's double meant she has outridden her four-kilogram claim which happens when an apprentice has ridden five winners.
It was a peach of a ride and one that the mare's trainer appreciated as it was her first win since February.
Stalker and Johnstone began their riding careers on the same day, so it was fitting that they each reached the first milestone at the same meeting.