That is correct, it is not an exact formula, however I have found providing a ratio for a medium viscosity all purpose mix seems to put people who have no experience at ease. If they have that, they have a starting point. Then if they prefer thicker, or thinner they can easily adjust. Mixing stone is no different, yet people always provide the melted milkshake referenced as a response on how thick. As for Rondo, never heard that term before today, and I have been doing this for a while. I am not calling you wrong. I made my conclusion on the thought that I would think that I would have heard of it over the course of 11 years. Bondo resin has been the only term I have heard over 100 people use in the industry, and my comment was based solely on that.