Cooking Varnish - Recipe & Directions

I am gathering ingredients for making varnish, according to Adam?s book appendix and from what I have read elsewhere traditional varnish was made from resin &oil and then thinned as needed. For the oil I plan on using cold pressed linseed oil, and for the thinner I am planning on using pure gum spirits turpentine. After much reading about turpentine it seems that the quality had gone down in the last 10 years. There are many forum posts and articles typically from luthiers and oil painters that talk about this. Pure gum spirits of turpentine is made from the distillation of sap tapped from living pine trees. Cheaper more common wood turpentine is made from shredded pine stumps, roots, and bark; this is not what should be used. Their advice is to order from a quality artist supply house and you get what you pay for.

I do have a few questions:
? What kind of resin should I use? I have read sandarac needs to be cooked at a very high temp to dissolve in oil. It seems that the most commonly used resin today is dammar. What about copal, rosin, mastic, or amber?
? Adam?s book lists an old recipe of 2 parts oil to 1 part resin. I have read that luthiers are using a 1:1 resin to oil, sometimes even less oil but again their instruments do not need to flex. Also any advice on how much turpentine to thin the varnish?
? Can I apply varnish directly over horn and sinew and skip covering the bow with birch bark and leather?

My plan for cooking is as follows, heat the resin until it turns liquid, add pre-heated linseed oil, and cook. According to luthier Keith Hill the varnish is ready when a little drop of varnish forms into a ball which, when you place it between your thumb tip and index finger tip and press can be made to adhere to both finger ends. When you pull your fingers apart you can produce a "gossamer thread". Before the varnish is perfect it will form a ball, it will stick to your fingers, but it will not produce that gossamer thread. Remove from heat and once the varnish is below the flash point of turpentine add in pre heated turpentine. Last strain varnish through a few layers of gauze into glass jar. Thoughts?