Victory at Twickenham on Saturday would see Leinster claim a third Heineken Cup crown in four seasons but Schmidt knows a repeat of last year's first-half showing will see them head back to Dublin empty handed.
"In last year's final, Northampton played superbly in the first half and we didn't take our opportunities," said Schmidt, whose team have won seven and drawn one of their eight fixtures en route to London.
"We made three clean line-breaks and gave the ball straight back to them. They dominated us in the first 40 minutes.
"We can't afford to do that with Ulster, especially when you look at their quarter-final victory over Munster: if it wasn't for a superb opening 20 minutes, Munster would have won comfortably.
"We can't let a team with that kicking ability, with Ruan Pienaar, Ian Humphreys or Paddy Jackson and Stefan Terblanche in their ranks, get a lead like that. Ulster are a team that can keep you under pressure because of their kicking game and they can also exert pressure on your platforms.
"John Afoa, Tom Court and Rory Best have done a great job for them in the front row; Johann Muller calls the line-outs and Stephen Ferris and Pedrie Wannenburg both add a lot. If we allow them to get a good start and build a lead it will be a pretty tough job for us to get back into the game."
One player certain to test Leinster to their limit in the English capital is Ulster blindside Ferris.
The Ireland and Lion star has been nominated alongside team-mate Pienaar, Leinster duo Rob Kearney and Jonny Sexton and Toulon's Jonny Wilkinson for the ERC European Player of the Year Award after another superb season for the Belfast-based province.
And while Schmidt can himself pick from an array of fantastic back rowers for the tournament finale, including Sean O'Brien, Jamie Heaslip, Shane Jennings and Kevin McLaughlin, the New Zealander knows stopping Ferris in his tracks will be as essential as starting strongly on Saturday.
"He had a fantastic Six Nations and there is a lot of respect from our loose forwards towards him. They have all played with him on occasions and speak very highly of him," added Schmidt.
"He's a complete player. He has a good offloading game, he attacks players, but knows when to pass, although more often than not he just makes a hole in defenders. He's also good in the line-out and adds some steel to their mauls.
"He makes a big impact in big matches - because they're the only ones he plays! They save him, wrap him in cotton wool and he comes back to play massive games."