I figured as much but couldn't do it until you told me it was possible! Thanks! Only two left.
The most annoying part of the very end game is just tracking down what portals you've missed. I finally realized that was the reason for the viewfinder, which is how I found my last missing portal. So you should be able to use that to find one portal you haven't unlocked yet.
But if you just want to know where that portal is....
If 11,9 is the one you're missing, then if you go to 11,15 you probably still have a disabled portal. If that's the case, then you should know where to go to teleport to it (and you should already know how since you had to do it already to use the other extra exit from the level.