Norris has scored points in every single race so far this year and only failed to finish in the top five once, including podiums at Imola, Monaco and the Red Bull Ring.
Fourth place at the British Grand Prix lifted Norris into third place in the standings above Red Bull's Sergio Perez. He sits five points clear of the Mexican driver, and is nine ahead of Mercedes' Valtteri Bottas in fifth.
McLaren F1 team principal Seidl was realistic about Norris's chances of ending the year third given the strength of the Red Bull and Mercedes cars.
But he felt it was a good target for Norris to have as he looks to repeat his strong first half of the year through the remaining races.
"Whether it is a realistic target is difficult to say, because the Red Bull car and the Mercedes car is really a better car," Seidl said.
"Valtteri and Checo also showed some great races this year and from the past, on what they can do. It is down to the strong performances by Lando together with the team and the competitive car we have this year and we managed after 10 races to be P3.
"So it is not unrealistic to stay there if he can put up this half of the season again like this.
"But we have to be honest as well one or the other at times something needs to go wrong on the Red Bull side or Mercedes side as we are not in a position yet purely down to our performance to beat them.
"That is the reality we are in and we cannot forget that."
Norris's result at Silverstone saw him break the McLaren team record for the most consecutive points finishes, marking his 15th race in a row inside the top 10 to eclipse Fernando Alonso's benchmark from 2007.
The British youngster has spearheaded McLaren's efforts this year as it fights Ferrari for third place in the constructors' championship. McLaren currently sits 15 points clear ahead of the Hungarian Grand Prix.
Seidl acknowledged that McLaren has its sights set far higher than P3 in the championship, but accepted that on current form, the most it can achieve is to hassle the leading two teams on occasion.
"Of course we wake up in the morning because we want to win races on the Sunday," Seidl said.
"That is not possible at the moment yet down to our own performance. So we will try to do better races in terms of strategy or car performance or staying out of trouble more than the others, or by luck from time to time.
"These are combined to put us in the position where we are at the moment thanks to, again, the great performance of the drivers and of the team here at the track and back home.
"That is what we will try until the end of the season, to give our competitors a tough time each time we reach another track."