Dude, you can get it down to 13 portals like this: In the room with the stairs, you can do that with 2 portals instead of 3, but placing one on the floor, on on the floor in the other room, turn off portal funneling, and jump through (same as what you have to do it lvl 15 advanced). That will make life easier when you reach the last room.

Thks ! Really interesting and not so difficult to do !