Cant help much on the score needed, I think if you asked their admissions department they'd give you a honest answer of what they're looking for. I assume that would then be caveat by how many apply (they'll likely take those with the better scores and 'assessment' results first) as Tormead Prep is very popular and usually full at intakes and I've heard has very few in year places avaiable.
On the school itself it's known locally for being, as a PP put it, 'lovely'. The Head and staff are well regarded, it has strong principled values around academics and value-add - but in the decades ive lived in Guilldford, its always been the school that has tempered this with actual kindness and wellbeing (not soundbites) - usp pastoral care.
Lots choose it as it's also a great route into the big school, which is also popular and regularly has a full year 7 - you skip the 11+ to move across Cranleigh Road to the big school- bonus - especially as the seniors get very good gcse and level results - also locally known for its pastoral/non-hot house feel.
Unless youre looking just inside Guildford there is also St theresas, which has always been a pretty decent option, but noises from people I know still aren't happy with direction of travel since the whole merger thing, thats still going on. May settle down though, so maybe worth a look.
StCatherines Bramley also pretty decent option, but depending on your location, harder to get to, although they do run a bus i think, but it depends on how long you want DD on a bus for, and for prep, its quite young for buses.
GHS obviously a academically strong, top tier school, but they've clearly been packing the prep school full this year, also with those who wouldnt normally have got in in previous years. Sign of the times perhaps.
Hope that helps -. Good luck with the choices theres plenty to think about.