You can also locate and consider the information in your child’s boarding school “college profile” as a starting point. Obviously you will gain much greater insight once the CC can engage.
I just pulled PEA as an example as one of many BS that don’t use a 4.0 scale.
You can get a sense of colleges students matriculate into, average test scores, and how the school will contextualize your students transcript.
Those kinds of schools usually have dedicated college counselors who are knowledgeable about historical admission and matriculation results from the school, and may have a good idea of what many colleges are looking for beyond stats, so they can advise students appropriately.
So start with the school’s dedicated college counselors, since they are part of what you get for your tuition money.