For some reason I can't edit my reply so I'm reposting with the edit....
And welcome to TU!! You'll find practically every (and I mean EVERY!!) answer to any question you could imagine here and you'll find all the folks here to be encouraging and very helpful. I know I have benefitted greatly by being here and my learning curve was flattened considerably.
By the by... I am new to soft plastics myself but to answer your question, I use 1/4 tsp to a 4oz. portion of plastisol. As far as plastisol goes, try everybody's stuff out before settling on one brand... Most folks will gladly send you a sample to try out.... I'm sure you're aware that there are several manufacturers out there, all with different grades (soft, medium, hard and more!) of plastisol... I am currently trying quite a few and have developed some pretty good ideas about what I do and don't like about each brand.... Personally, I prefer softer plastics as I feel they have better action (at the potential cost of durability, but if I'm only catching one fish per worm I'm ok with that...) but with other lures that I will be making in the near future I believe I'll find that a firmer plastic would work better.... Like I said, try as many out as you can...