^IMO not another layer of BC. Go for black. My theory is that the BC10 is already doing most of the heat rejection. Adding a black backing would just augment that rejection, plus kill off some of the reflection of magic tint. Try sampling both 35 and 20 on your windows and see which suits you.

Personally, mas sulit ang 20. Just got myself that on the Prado's windows this Monday. Ironically, it's not been as sunny or hot. Hahaha!