PANS-OPS says:
3.5.5.3 The width of the ILS/MLS final approach area
is much narrower than those of non-precision approaches.
Descent on the glide path/MLS elevation angle must never
be initiated until the aircraft is within the tracking tolerance
of the localizer/azimuth. The ILS obstacle clearance surfaces
assume that the pilot does not normally deviate from the
centre line more than half a scale deflection after being
established on track. Thereafter the aircraft should adhere
to the on-course, on-glide path/elevation angle position since
a more than half course sector deflection or a more than
half course fly-up deflection combined with other allowable
system tolerances could place the aircraft in the vicinity of
the edge or bottom of the protected airspace where loss of
protection from obstacles can occur.
That doesn't answer the question you asked, but does give an idea of when you're at risk of hitting something.