I really expected this comment, and it is a valid one.

If we remember, the S-75M Volhov maximum killing range is...
Vtarget<Mach1 - 56km
Vtarget<Mach2 - 43km
Vtarget<Mach3 - 40km
... depending on target speed, so you are correct.

But this is valid only if the missile-target speed range is comparable.
(Missile 2.6~4Mach, target 1~3Mach)

With the S-300 the missile speed is (6Mach) well above the expected air breathing target speed (4Mach).

In case you have ballistic missile target (9Mach), your range would be again (similarly) reduced to 40km from the original 150km.