First, eliminate the word "fair" from your vocabulary. Fairness is relative and by definition cannot be measure objectively. What is "fair" in your mind may not be "fair" in the B&B's mind, and you could end up missing out on a great opportunity just because your feelings and sense of "fairness" got hurt.
Decide what your time, effort and talents are worth, and ask that they pay you that amount. Oh, and that is not a %. A % is not an amount, it is a %, again a relative things. State your wage in $, an objective measure.
Sure, what local spas charge/pay is a ballpark, but you will find a different situation in nearly every place, so there is not an apples to apples comparison. The MTs from the local spa may be doing it "off the clock" from the spa, and people tend to have lower pay expectations of "gravy" vs. their primary source of income (how many PT jobs pay more than FT jobs?...hardly any...if they did, they would become your FT job!)
Besides, it is not about what other people make or don't make. That won't make you happy. Only what you earn can satisfy your needs.
It is about you, and your skills, and your effort and your willingness to work. Don't make experience a factor either way, because the guests won't know or care how long you have been doing this, they just want a top quality treatment.