Not picking on accountants but they can be prone to be overly cautious. Matching is 7 1/2 percent. There may be another point or two in your state to account for unemployment and such. While the 30% is mostly spot on, most of the rest is subjective. I say this because often folks add the 30% in their budget but they never actually compensate their employees at that level. You have to consciously put together a package that pays at that level.
Here's a nice link that sums up what the averages are for everyone.
Simplyhired.com has this to say about potters in general:
The average salary for potter jobs is $48,000. Average potter salaries can vary greatly due to company, location, industry, experience and benefits. This salary was calculated using the average salary for all jobs with the term "potter" anywhere in the job listing.
Where you fall in there is obviously based on your shops circumstances. You mentioned that you have a lot of experience hiring and that your looking for what is customary with artist and I think mostly they don't fare well in the compensation arena.
I guess I would make the plug that although an aspiring potter may not generally be able to command high salaries and often work long hours for low pay simply out of love of the job, there is a lot to be said for being generous with employees. It really builds a strong team that is going nowhere unless they reach a point they want to strike out on their own. Some do, but the vast majority of people would prefer a secure job with a company they are both proud of and feel very connected and appreciated. Pay good livable wages with regular bumps and an array of benefits such as flexible time, two or even three weeks of vacation, ample sick leave and at least a major medical insurance policy and you will build a very strong team that will dig deep when they need to and put your company first. Artist are no different than the rest of us and they have to pay all the same bills each month.