what is the lv of the equip you need to disassemble to make ...

intermediate item crystals? the lv. 70-90 ones. Do we just need to disassemble lv. 70-90 equips?

Yes and I'm pretty sure it says that on the description of the item crystal :|.

you can dissassemble lower-level equips too though, there's a chance they'll give the next rank of crystal.

As Shinku said, if you disassemble 40-60 equips there is a slight chance you can get Intermediate crystals.

so if you disassemble a lv. 70 equip, it will 100% give you a intermediate item crystal?

so if you disassemble a lv. 70 equip, it will 100% give you a intermediate item crystal?

There's a small chance of getting advanced item crystal.

You can also disassemble the level 64 maple weapons for the intermediate item crystals. They are in between the basic (60) and intermediate (70) range, but count as intermediates as I got a whole stash of them from Golden Temple and never any basics.