That 25-24 win last Friday night was only their second in 11 years at Stade Aime Giral and Johnston believes that was the perfect preparation for facing Biarritz at Estadio Anoeta.
"We could not have asked for better result heading into the last eight of the Heineken Cup," said the giant Pacific Islanders prop.
"We had not won in Perpignan for such a long time and it's the sort of place that some people don't get out of still walking.
"We were bitterly disappointed with the result in Paris a week earlier against Racing Metro so to win at Perpignan was a massive boost for our confidence and we can take that into the quarter-finals and another tough away match."
The Samoan prop knows all about playing inside the cauldron atmosphere of Estadio Anoeta having been part of the Biarritz team that beat Bath in San Sebastian on their way to the 2006 Heineken Cup final.
And he has warned his Toulouse team-mates of what to expect in a repeat of last year's final.
"We know we cannot afford to let Biarritz make a good start because then the challenge will only become tougher," he said.
"San Sebastian is the sort of place that any opposition dreads going to. The crowd and the atmosphere they generate give the Biarritz players a head-start from the kick-off. I should know because that was how I felt when they were cheering for me.
"That is something that we will have to try and block out if we're going to win. We have areas of our game that we can improve upon but the first thing is dealing with the crowd. If we can keep them quiet then that will be a good start.
"However, Biarritz have so many top players. Obviously they have the big French internationals like Imanol Harinordoquy, Jermone Thion and Damien Traille but they also have great club players like Takudzwa Ngwenya and Dimitri Yachvili who give everything to the cause. There is no doubt that we will have to be at the very top of our game to win there."
Johnston wiped away the painful memory of losing in a Heineken Cup final by lifting the trophy with Toulouse last year in Paris. However he has the chance to lay to rest the ghosts of 2006 in the Millennium Stadium when the final returns to Cardiff next month.
"Cardiff has not been a great place for me, either with Biarritz, Toulouse or Samoa," he said. "But that doesn't mean I don't want to go back there again. However, there is no way we can look any further than next weekend.
"Winning the Heineken Cup is all about taking little steps. We suffered a real set-back in the last game of the Pool stages against Wasps which cost us home advantage in the quarter-finals, but winning at Perpignan last weekend was definitely another step forward."