Some harissa pastes are spicier than others. If you want a milder tofu, start with half the amount and taste before adding more. For extra spice, add a bit more harissa, but mix the sauce and taste as you go.
Important Note about Salt: Not all salts are made equal and cannot be substituted 1:1. If you are substituting table salt for kosher salt, you will need to cut the amount called for in the recipe by at least half.