I don't use MPS at all in my tub, but it can be helpful after heavy bather loads. When I have been gone for several days, up to a week, I have raised the FC to 12 ppm and lowered the temperature. Came back to several FC in the tub each time. As long as you don't drop to 0 FC, no harm to sanitation but aim to keep it at at least 2-3 ppm before your next dose. With no use, 2 ppm per day FC loss is normal, could be higher at higher temps. If your CYA is at 30 ppm, it's not harmful to you or your tub at FC up to 12 ppm. You can use the same relationship as a pool from the CYA/Chlorine chart in my signature for shocking your spa after use or to elevate FC when unable to check for a couple days. Spas can be a bit more of a hassle to maintain because of the higher temperatures than most pools and the much higher bather waste load relative to the water volume.