I don't really know the answer to this, but you do know that you can't take the WJ-III grade equivalents literally, don't you?
With something like Math, the best way is to take 'end of chapter tests' when you start to eyeball that material could be somewhat challenging. That would work with Spelling too.
Language Arts seems to be more nebulous - try to find teaching materials that give you an overview of reading and writing development, Such as '6+1' Writing Traits. Good news it that the more one writes, the better one gets at it. I've never hear anyone say, 'we introduce Simile in 5rd grade so that we can teach Metaphor in 6th grade.'
Of course to me, the 'Unit Studies' approach is very appealing. If a kid has a lot of motivation to study 'cars' for example, why not teach the Industrial Revolution, Math that has 'miles/hour' and other Stats,books and movie and writing assignments about cars, Global Warming.
My son once spent 45 minutes 'making graph paper' because he was interested in a Math idea (and I, Queen of where things are kept, wasn't home - this is a DH story.In fact it is the story where DH finally started to accept that I wasn't making the whole thing up.) It was great for his fine motor development.
For Science and History, one approach is to take a look at Middle School Level Texts and use them for jumping off points.
Hope that helps -
Grinity
