Before I started this thread, and was playing with this in my head, I just wasn't getting anywhere, but after making the graphic below (specifically for this thread) and measuring the liquid content to each of the three level marks, I think I have worked it out, but I might as well run it by some of you young folk with still function brains. 
The graphic represents the reservoir from my Bex Bissell carpet stain cleaning machine. The chemicals are fairly costly, but it is known that what they call the Oxidising Boost agent, is 5% dilution of Hydrogen Peroxide, and you can get it for a tiny fraction of the cost from the likes of eBay. Except they only provide it in 3% dilution. So I need to work out where to put the new mark.
It uses 220cc of the H2O2, which I worked out to be 11cc of pure H2O2.
Together with the 700cc of water, that's 11cc in 920cc , and I need to achieve that with my 3% dilution H2O2.
I think it is actually really straight forward, but I would like to hear if I am getting this wrong.
If 100cc of the 3% solution contains 3cc of pure H2O2, then I need 11 / 3 = 367cc of the 3% solution.
So I just need to subtract 367 from 920 = 553cc and make a new mark for the water level.
Add 3% solution to the original 2nd mark, and then add the 100cc of the Spot & Stain chemical as normal.
Incidentally, we were spending serious money getting the commercial cleaners in every time one of the cats ejected a fur-ball, but this machine is every bit as effective as their steam plus top secret chemicals that they are not allowed to tell you about. Probably because if everyone knew how simple it is to do it yourself, they'd be out of business.
On the downside, after ordering my two litres of Hydrogen Peroxide for the cleaner, I read that hydrogen peroxide, together with acetone (nail polish remover) can be used to make a high explosive called triacetone triperoxide (TATP), which is used by ne're do wells do perform dark deeds. So I am probably now on a government watch list. 
Acetone peroxide - Wikipedia I find this serious disturbing — not about being on a watch list, but that a powerful explosive really can be made from household chemicals.
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What hit me was my incredible naivety. I mean you read about Jason Bourne trapped in a strange house, and diving under the kitchen sink and coming out with a hastily contrived device that blows the house apart just as his pursuers enter, but I have never taken that sort of thing as genuinely doable.
With curiosity properly piqued, I could help but look into it. I had convinced myself that the 60cc of pure H2O2 you'd get after distilling two litres of 3% solution, was not going to be a significant quantity. Apparently I was wrong — this 'experiment' used just 30 grams! You need to turn your sound way up to hear it properly
Another ten minutes on Google told me that TATP is definitely not easy to make, and that it is extremely volatile and dangerous. So probably something that not even Mythbusters would mess about with... or would they?
jane-e wrote
Was that your home video Trevor? WOW!
Good gracious no Jane 
. It was curiosity leading me to see what on earth the Hydrogen Peroxide mix was all about, but taken no further than ten minutes with Google. Another quick look today tells me that TATP is well known to the bad guys, and volatile and likely to go BANG before you are ready to such an extent, that it carries the nick name (in Arabic) of 'The Mother of Satan'.
Anyone tempted to play at making their own fireworks with those chemicals should have a look at this page. The weird thing is that some of the comments appear to indicate that despite the huge danger, some people are still tempted to 'have a go'. Perhaps we'll read about them in the next round of Darwin Awards!
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