You are correct Zeezeemum, very little published information appears to exist on out of area intake for SHSG. From admissions criteria supplied with the 2011 prospectus “Up to an agreed maximum 115 (out of 150) places available for local schools provided they satisfy min test score” They also go on to state “pupils from outside the local area are, however, encouraged to apply as the School always admits significant numbers from outside the local area”
As an example figures for WHSG entry for the 2006/07 academic year showed that 98
(out of 154) pupils from out of the local area actually got in. Even allowing for local/annual variations, it's safe to say a lot more than 35 (150 -115) pupils will get in SHSG from out of area.
Perhaps would not worry to much about number of places. The key point, as a general guide, is to target an average 65% exam pass mark (VR counts double). That will almost certainly guarantee an out of area place.
Hope this helps and wish your DD the best of luck.