Chlorine Demand is the single most important measurement you can make, to determine the health of your tub water.
Any organic "stuff" (dead or alive) in your tub, Chlorine will kill and oxidize. The more "stuff" in your tub, the more Chlorine is needed. The question becomes, is there a measurement of how much "stuff" there is in your tub. Yes, Chlorine Demand (CD)! CD is basically how much Chlorine (sanitizer) your tub is using. This also applies to Bromine. CD will tell you how much Chlorine is being used, and therefore tell you how much "stuff" is in your tub.
Now that we know what CD is, how do we measure it? That's the easy part.
1st, Shock your tub to around 10ppm FC, and make note of where it's at.
2nd, Cover and let sit for 24hrs. DON'T use the tub!
3rd, The next day, test FC again.
Now, take the 2nd measurement and divide it by the 1st measurement. This will give you a percentage of how much Chlorine is left in your tub after one day. Obviously the higher percentage the better. 100% means your tub is not using ANY Chlorine (not likely). 0% means your tub used ALL the Chlorine you put in it (NOT GOOD).
To take it one step further you can calculate CD, which is One minus the Percentage you calculated. That will give you the percentage of Chlorine your tub is using. Obviously the LOWER the better. If it's 0%, that means your tub is using 0% of the Chlorine (again not likely). If it's 100%, that means your tub used 100% of the Chlorine you put in (again NOT GOOD).
Measuring CD is the easy part. Interpreting it is a little trickier. It's best to first measure CD on a fresh refill, BEFORE you use the tub. That way you have a good Baseline (BL). My BL is ~25%. That means, when I shock my tub to 10ppm FC, the next day it is ~7.5ppm. After you start using your tub, the CD will increase. The trick is to try to keep it as close to your BL as possible.
For example, let's say you use the tub (4 people for an hour). After you get out, you shock the tub to 10ppm. The next day you check FC and it's 2ppm. That means your CD is 80%. That's too high and means you didn't use enough Chlorine the night before. Not a problem, just shock to 10ppm again. The next day check your FC. It should be above 5ppm. If not, keep shocking until it is. If you're not using enough Chlorine, your CD will increase and be more difficult to get back down.
So what's a good CD then. Here's my recommendation.
~25% is Ideal. This should be your Baseline.
~50% is Ok. This is probably where most people are at.
~75% is Poor. This means "something" is using up chlorine too fast. You need to shock.
100% is Bad. This could mean you just had a Hot Tub Party, you've been using way too little Chlorine, OR you have "something" actively growing (a bug) in your tub. The former two mean you should start shocking. The latter means you need to decontaminate your tub.
NEVER let your CD get to 100%, or you'll have problems. Also, the older your water gets, the more difficult it will be to lower the CD. If you find it difficult to lower your CD, it could mean you're due for a water change. It could also mean your filter is dirty and needs to be cleaned. Make sure your keep you filter clean, or it will use up Chlorine. Lastly, MPS will help Chlorine oxidize waste, and therefore lower your CD. So if you're using MPS, you may want to wait a few days (week) before you measure CD. If you find it difficult to lower CD, try shocking with MPS.
Hope that helps you keep you water cleaner.