The winger, who was among the scorers against John Hughes' side, said: "It was a good high. We obviously wanted to win but to do it so convincingly was extra nice. We're feeling confident right now and the games can't come quickly enough for us.
"I was chuffed with my goal. I've set myself a target to score more this season. I've think I've got nine so far and there are definitely more goals in me.
"I feel that with the chances we are creating there's goal to be had in every game. I've set myself a target of 15, so I'm getting there.
Mackay-Steven, who had two years at Liverpool as a teenager, still hopes to make it in the English game but feels he is thriving at Tannadice for now. The 23-year-old said: "I feel I'm really improving my game under Jackie McNamara, Simon Donnelly and Darren Jackson.
"They've helped me so much by developing different parts of my game. I think every player wants to go as high as they can in the game and I think England is the best place for that. That's the aim for the future but right now I feel good things are happening here."
United are fourth in the table and Mackay-Steven believes they can still challenge for a place in Europe. A win tonight would put them just four points behind third-placed Motherwell.
"I think that's still a realistic target," he said. "We had a blip in December that put us back a bit but we were always confident we would get back to winning ways.
"That's happened in the past few weeks and if we can keep the momentum up then hopefully it can take us into contention for a European place. That is definitely the aim for us.
Ryan Gauld could return to the United squad after recovering from a hip injury. "Everyone came through the cup game okay and Ryan trained with us on Monday," said McNamara, the manager.
"We will see how he reacts before deciding whether to put him back but we are delighted to have him back in training. Unfortunately, it looks like Curtis Good [hip injury] will be out for four weeks."
On his team's form, McNamara added: "It is going well at the moment but we are just taking it game to game and we have another massive one against St Johnstone.
"They will want to try to close the gap on us while we want to close the gap on the teams above us so it should be an entertaining match."