J.C.
You will get your best answer by going to
www.rhtubs.com and addressing your questions to Northman and/or Vermonter. They are experts in this field of spa chemistry.
Northman has written extensively on why calcium hardness is unnecessary in modern hot tubs. He runs tubs with soft water. He says only if you have a concrete pool or spa or metal parts do you need to raise calcium hardness, primarily so that the calcium in the concrete is not leached out and metal is not corroded (and only under low pH conditions below 7.4). See the FAQ section at that web site for details.
You will get scaling at pH above 7.8 with the levels of calcium above 90 ppm. If you meticulously maintain pH at 7.5 with TA between 100-130 ppm, there will be no problems, even with soft water (calcium below 50 ppm). The advantage of soft water is that scaling is virtually impossible.