The midfielder helped his side claim a 2-0 victory in Sunday's first leg at Whaddon Road, Jermaine McGlashan and Ben Burgess finding the net.
That puts the Robins in pole position heading to Plainmoor on Thursday night but the scoreline is a precarious one. And Pack, a mainstay in the middle of the park for Town this term, knows the opening goal is massive - whichever way it goes.
He said: "Going into the first leg I think anyone would have taken a 2-0 lead in a play-off game where things are relatively tight and hard to come by. But I think the game on Sunday was quite open and frantic.
"As I say, we would have taken that all day long but we're going to Plainmoor and it's a tough place to go, they're going to be up for it from the first whistle."
Pack added: "Even in a normal game, a 2-0 lead is quite tough to keep hold of, one goal back and it's game on. I think maybe the first goal on Thursday will be the crucial one of the evening and probably tend to see where the game ends up. Hopefully it'll be us with the first goal and we can kick on and see the game out."
