Say the number, during
Out of ten. Seven is what you are aiming for: distinctly strong, unmistakably work, and you can still breathe normally through it. If your breath has gone shallow or you are holding it, you are at nine, and nothing good happens at nine.
Say it while it is happening rather than at the end. "A bit lighter through here" costs you nothing and changes the next eighty minutes. Told afterwards, it only informs a session you have already paid for.
Why nine does not work
At nine your body guards. The muscle contracts against the hand to protect the joint underneath it, which is an involuntary reflex and not something you can decide your way out of. So the practitioner is now working against a muscle that is actively resisting, at a depth it will not yield at, and both of you spend the hour losing.
The soreness afterwards is not the work landing. It is the bruising from an hour of that argument.
Deep is a tempo
The phrase "deep tissue" is badly named and it costs people a lot of comfort. It does not mean harder. It means slower, and aimed at a layer beneath the one most hands work. Pressure applied faster than tissue can yield produces guarding; the same pressure applied slowly at the same depth is routinely described afterwards as gentler, and does considerably more.
If you have been told you need "the strongest person in the building", you have most likely been having fast massage rather than deep massage. One session tells you which.