If you want to use a GVG I would suggest keeping the amount of plant matter to a minimum. I would suggest a 1:1 ratio as it is easy to vape the whole hit on one go this way. I have also found that being a little heavy on harmalas with vaped Changa is a good way to go since it vaporises at a higher temp than the spice - so some may be left behind unless you are thorough with your smoking technique. Personally I am not a massive fan of vaped / smoked caapi leaf, I would use mint and passion flower out of the ingredients listed, but part of the fun of Changa is putting together your own blends, so PLEASE do experiment!
I would start off with doses containing around 15-20mg Spice from the GVG it is highly efficient and I was dazzled by my first 15mg from this contraption, but by all means go a bit higher if you are feeling adventurous (just don't be silly lol)...
for the blend I would personally go for something like this:
200mg Spice
200mg Leaf
20-50mg Harmalas
Lose Control, Free My Soul, Break Me Open, Make Me Whole."DMT kicked my balls off" - od3