I kept a log for three weeks: grind size, tamp pressure estimated on a bathroom scale, shot time, and taste notes. The surprising result was that consistency mattered more than the actual pressure number. Shots tamped at a wildly inconsistent 15 to 40 pounds pulled worse than shots tamped at a steady, unremarkable 25 pounds every single time.
Once I started tamping to the same firm click every pull, channeling dropped off almost completely and shot times stopped swinging by ten seconds between cups.
Buy a cheap tamp scale if you want, but the real fix is muscle memory, not a magic number.