Hi Mark, the ideal mortar mix is dependent on the space between the aggregates, the sand. You can measure that space easily. Take a sample of the sand and bake it to drive off the moisture. Place the sand in a known valume container, a liter for instance. Pour in water, measure the volume of water, until the water level reaches the top of the sand. The water volume represents the ideal volume of lime. Develope a ratio of lime to sand by volume. When mixing mortar, keep the ratio constant by using a bucket for a measure rathe than counting shovel scoops.

Use Type S Hydrated Lime and sharp builders sand. I would avoid silty river sand.