A 5% reduction in main rotor thrust will cause about a 20% reduction in TR thrust.
Surely the TR thrust reduction follows a square law? So assuming a nominal 100% Nr is normal, 90% Nr = 81% TR thrust, 80% Nr = 64% TR thrust etc.
it doesnt have the same inertia as the head
Why is this important? The TR is driven from the MGB and is not in autorotation, so its speed is entirely a function of what the Nr is doing - where rotor inertia is obviously a factor.