Karlsson @ 10M = 14.7 percent of a 68M internal salary cap
Karlsson @ 12.5M = 18.4 percent of a 68M internal salary cap
Karlsson + Bobby @ 17.5M = 25.7 percent of a 68M internal salary cap
Karlsson + Bobby @ 20M = 29.4 percent of a 68M internal salary cap
That's assuming our internal budget does not rise as it hasn't risen over the last few years despite the cap going up. Bobby lumped in there based on the opportunity cost of dumping Bobby in a Karlsson trade that is lost by extending Karlsson.
Make of those percentages what you will....
In a fantasy world, if I were in charge of a cap ceiling team, I wouldn't have a problem with paying Karlsson max (16M this upcoming July 1st) if there were no bad contracts already on the books. Obviously, I'd ideally want to get him for less, and it probably won't take max to extend him. Sens aren't a cap ceiling team, and they have oodles of bad contracts. In the Sens shoes, unless there are major changes, I trade Karlsson.