Standardised scores are based around the average being 100. Anything between 85 and 115 is considered being in range.
Someone with 85 though may struggle is the rest of their standardise scores are low as they likely don't have a strong skill to use to counteract a weakness.
Anything below 80 is considered poor and below 70 is severe.
My ds spelling is below 70 and has always been despite inputs. He just does not get/hear or something phonic sounds and syllables.
His reading is 85 and although lower end of normal with his asd and lack of inference skills and higher order language it all comes together to make his literacy poor.
Percentiles can be misleading imo. Being 50/60/70 percentile is fine. It's joe blogs average. It means that if you took 100 children the exact same age 50 have weaker skills and 50 stronger or 60/40, 70/30.
There's no reason someone on 50th centile can't learn to the same level as someone on 90th - they may just need to work harder iyswim?
However there's also the possibility that someone on 50th centile will struggle if they have other LD alongside it meaning they can't find an alternative method of learning or have another skill to counteract it.
Likewise someone with a 90th centile score in one area can still have a LD.
Example is: my visual memory is 2< centile. However my audio processing and recall is on 99th! Therefore I can just have it read to me (many computer programmes to do this) and I'm fine.
If my audio memory was also 2< centile I'd have bigger problems!
I've probably confused you more
but it's more about how these cognitive tests inform towards a way of learning or a way of teaching skills rather than them being able to predict an academic ability iyswim?
I really do think you need to get back to the EP and ask for clarification.
Just email the la and say you need longer as it's not meeting g legal requirement of specific and quantified currently and you need the professionals to provide information.
Get the mediation certificate in the meantime.
And consider a private EP report if it's financially viable. I've heard of judges not agreeing to parents saying he needs "x y and z" because it's not stated in reports - even though it should be.