A level was setup exactly at the middle distance between staffs A and B that are

$70\mathrm{.}00$ meters apart. The reading for staff A was $1\mathrm{.}550$ meters and for staff B was $0\mathrm{.}930$

meters.

Then the level was moved and setup touching staff A $($i$.$e$.,$ the setup level is $0\mathrm{.}00$ meters

from staff A and $70\mathrm{.}00$ meters from staff B$).$ The reading of staff A was $1\mathrm{.}900$ meters and for

staff B was $1\mathrm{.}200$ meters $($staffs A and B were not moved during both settings$).$ What is the

error per meter for this level, if any?