I'm trying to find a simple yet accurate way to figure out how a certain percentage of white ink (only white ink) printed to a clear substrate correlates to the percentage opacity that white ink layer has.
The layer in your artwork will naturally range from 0%-100% opacity. 100% in your artwork will correlate to the maximum opacity achievable for the given print method and the ink used. This maximum opacity varies a lot, but for most processes is probably lower than you’d expect.
E.G. Flexo white is normally only something like 70-80% opaque. There are ways to increase this such as applying two hits of white ink or mixing an extra opaque ‘super white’ ink, but these methods incur extra cost for diminishing returns. For instance, two hits of white might only increase your opacity from 75% to 85% while costing more than adding an extra colour to the design. Again, surprising but true. White ink costs more than any normal colour because it contains a high amount of titanium dioxide which is expensive stuff.
All that said. Once you know the maximum opacity that your printer can achieve, the relationship between artwork and print opacity is pretty linear. So if your max opacity is 70% then 100% in your artwork will be 70% when printed and 50% in your artwork will be roughly 35% and so on. Zero is obviously always zero, but if it is flexo printing you will be restricted by the minimum printable dot.
Most printers will know what opacity they can get or will be able to run a trial on the right substrate and supply samples for reference.