So, Bob McKenzie has a "quick paragraph" on how it's supposed to work...
From: http://www.tsn.ca/nhl/story/?id=299097
Tagging room is calculated by totaling the dollar values of the contracts that will expire at the end of the season plus the club's available salary cap space that exists today. Whatever that figure is becomes the maximum amount a club may commit to contract extensions for next year.
He goes on to say the "available" number seems to be around $18M and the total amount of the extensions (Kane, Toews and Keith) seems to be around $19M. So, they have to "fix" it so that those two numbers match. If it's that close, it should not be a problem. And they just may have to wait a bit before announcing it.
To be honest, the tagging thing seems silly. I say let them sign whatever extensions they want, even it they go over the cap for next season, just make sure they're under the cap at the start of next season.
But that's just me.