Hi Tom
The question you propose is interesting.
I see some information on the internet that some use 50% vinegar in relation to the total amount of water and 2% salt and 3% sugar.
I think that, as you say, the proportions could change depending on the ingredient we are pickling. I also think that with practice and experience you learn how to handle pickles and their proportions.
We are going to wait for the opinion of other colleagues who have experience in the topic of pickles and can help you by giving you more information on your question.
Greetings.