I would fill in your CAF with your preferred school first.
SGS has (in the past) sent out its own letters offering a place as well as the LEA offering you your highest placed school that you are eligible for.
If you put say SGS first and he scored 118 then he would be allocated SGS
If say you put Herschel first and SGS second , your son scored 110 for Herschel (not eligible) and 118 for SGS then again your son would be allocated SGS.
If you put Herschel first and your son scores 120 for Herschel and 118 for SGS then you are eligible for both schools and will be allocated your first preference..... Herschel.
If more than 200 people put Slough Grammer as there first choice and were eligible to get a place how do you pick
The allocation should be done on scores and operate an 'equal preference' system.
SGS offer to the highest scores first (It doesnt matter where on the CAF they are put)
If the child has also qualified for a higher preference school then the LEA will inform SGS and work down the list so that each child should get the highest pref school that they qualify for.
There are subsequent rounds of offers as people (sometimes decide to go private) turn down offers. Therefore you may be offered a higher preference in the months following allocation day.