"I will take home some lessons learnt from this," said Johnstone, who was a member of the bronze medal winning team at the 2010 Alltech FEI World Equestrian Games in Kentucky.
"Less would have been more in terms of the warm-up for the dressage. This is a sport that you never stop learning. There is a lot to take away from the weekend - mostly positive but a few things to work on."
The 11-year-old grey gelding was allowed as many treats as he wanted - mints, carrots, apples and more.
"He is like a Labrador when it comes to begging for treats."
Johnstone took his hat off to Rose, who led from the start.
"I am very happy with third, but first would have been better, however, Shane has gone very well and he was the better man on the weekend."
Johnstone is eyeing a spot on the New Zealand team for the 2016 Rio Olympics and next week will meet with the high performance eventing coach Erik Duvander and high performance director Sarah Dalziell to plan his next eight months.
"Rio is most definitely the goal."
In the CCI2*, Australia based Kiwi Hayley Frielick and My Happiness finished in 14th position after dropping two rails in the showjumping phase to have a final score of 67.4.
The class was won by Amanda Ross aboard Discavalli Diesel who led from the start and finished on 44.8.
Frielick was also 33rd with her other horse Class Action LP.
Laura Wallace and Sleeping Tiger finished in 44th place.