MORTAR SPOTTER. Any attack made by a German Mo rtar may
have its LOS traced from Sergeant Biermann inste ad of from
the Mortar itself. This activates Biermann for the Turn. The
Targeting Roll must still use the actual range from the Mortar
to the target hex but any Hind rance modifying the roll will be
based upon the LOS from Biermann.
This is how we have been applaying it so far, litterarly following the SR as written.
(a) I activate the LM carrying unit for Fire.
(b) If Biermann is not activated, I announce I'll trace the LM's LOS from him and activate Biermann as well.
(c) I proceed as written.
Very restrictive reading of the rule.
I think you have it correct except for one possible deviation in step (a).
The way you wrote step (a) indicates to me that you are using a fire order to activate ONLY the unit carrying the mortar (Yes/No??).
The SR states that "any attack made by a German mortar....." This indicates to me that the unit is activated to fire, but that activation could just have easily come from a different leader using his command radius to activate multiple units (including the mortar team).
If you are limiting yourself to burning a fire order just to activate the mortar team for Biermann to spot for, I think you can untighten that restriction.