I think 6.5 as a ceiling gets it done.
There aren't many goalies making 8M+. The writing is on the wall in Boston. Additionally, he plays the most volatile position in the sport. If Ottawa is ready to give him 6.5M x 6 to 8 years with a NMC, front-loaded money, and signing bonuses - it would be very risky to turn that down and either stay in Boston (where he won't be #1), or play out the season somewhere else behind a new system.
Of course, I am assuming we can offer him a competitive contract term wise. If we're still in the same boat that we were with Melnyk, and we cannot do thinks like signing bonuses or front-loaded money, then it's going to take a higher AAV to offset that.
6.5M would make him the 5th highest paid active goalie in the league. He would be ahead of everybody except for Bobrosvky, Sorokin, Vasileskiy, and Hellebucyk. Those are all guys who were perceived as franchise level goalies when they signed.
Then you add in that not only would he be above all but 4 goalies, a chunk of the other goalies near him wouldn't have the kind of terms that I am suggesting (signing bonuses, NMC, etc).