Here goes my unscientific, totally intuitive calculation:

1 bag sand = 50 lbs; 1 bag Portland = 94 lbs.
So, 1 bag Portland is approximately 2 bags sand. That would require 6 bags of sand for the 1:3 ratio.

This also fits my visual volume approximation that 2 bags of sand = 1 bag Portland.

David

Puhleez correct this, someone, if there are factors unknown to me that need to be considered. I made A's in geometry and D's in algebra in high school. Logic is not necessarily a sincere friend when one doesn't understand the chemistry.

In hypertufa it seems like talent, luck and the phase of the moon can alter results.