The sand/lime ratio does depend on your sand. It is worth doing a series of tests 1:2, 1:3, 1:4, 1:5, etc. Spread on a board or a bit of wall. Watch and see what happens when it dries. Too hard and cracking, or too soft and crumbly. I hosted a Tiez Breizh course last year and they rendered my walls with one part lime to 3 parts washed and 3 parts unwashed sand, i.e. 1:6. The clay content of the sand affects the mix, and sand is cheaper than lime. And sand comes in lots of colours.............Lime wash plus a bit of pigment is a cheap and easy paint finish.