Hmm. Maybe I didn't notice there was a slider to control the lid angle... Let me take a look at it again.

EDIT: Ok, there it is. Never mind then about my previous posts. Seems like your ggb setup can do what I was talking about. I missed the slide to the left of point "A" the last time I looked at it.

Now I just have to play around and then multiply out the ratios to calculate real-world dimensions. I'll give it a first shot later tonight I hope - I'm probably going to spend most of the day outside today since the weather is amazing right now.