Someone asked why I fudged the proportion of varnish in my medium recipe.
Varnishes differ widely from one another. Some are made with oil and some are made with thinner. Some start concentrated with the expectation that they will be thinned later, while others are already thinned to some desired consistency.
Damar, which along with mastic is one of the soft resins, is still widely used by artists to make varnish. Once common varnish-making technique is to dissolve the damar crystals in turpentine in proportions 1:1, that is, one portion of crystals to a similar volume of thinner. After the crystals are dissolved, the resulting concentrate is thinned again before use. Recipes vary from 1:1 (1 portion of concentrate further thinned with 1 portion of thinner) to 1:4 or more. So, there is a lot of variety even with this simple varnish. The picture grows more complicated still if the damar crystals are melted in oil.
Copal, along with amber and some others, is a hard resin. Because the hard resins are more difficult to melt than the soft ones, hard resin varnishes are infrequently made by artists.
So what all this means is that the proportion of varnish in your recipe varies depending on the varnish. A thick, concentrated varnish requires more thinner.