There is no magic number and the 10x rule is wrong anyway. Somebody in the past forgot that there is a factor of 5 unit difference between ammonia in ppm N units and chlorine and chloramine (including monochloramine) in ppm Cl2 units and they also forgot that CC will already have chlorine attached to it. The 10x rule came from chlorine oxidation of ammonia, not chlorine oxidation of CC. I'd ignore much of what you read unless it's from a scientific peer-reviewed paper in a respected journal. Higher chlorine levels will oxidize chemicals faster, period. If you want to superchlorinate to any very high level you want, that's up to you. You could do a test with a bucket of spa water if you want to use less chemicals just to see IF this CC will go away with chlorine at all and if so at what level and how long it would take.
It is true that there can be persistent organic chloramines that are slow to oxidize from chlorine, but in a hot spa with residential bather load this is very unusual. There are definitely chloramines that show up after bather load, but usually within 24 hours, again at HOT spa temperatures, they are gone down to <= 0.5 ppm. Yes, you can just replace the water, but it would be good to know what caused this in the first place. Otherwise, you'll just have it happen again and not know why -- perhaps some sort of chemical on the skin (lotion, etc.).
An ozonator may help, but ozone doesn't oxidize everything so it depends on the specific organic chemical that is causing the problem. Odds are it will help, but there's no guarantee. Enzymes might help as well, but again, no guarantee.