You'll catch on. If you use the 10ml line you may (for example) get a reading of 10, 10.5 or 11. If you use the 25ml line you may get 10, 10.2, 10.4, 10.6, 10.8 or 11. 10 and 10.2 aren't significantly different, neither are 10.8 and 11. And 10.4, 10.5 and 10.6 all pretty much the same.

So you should get the same reading but the 25ml is more precise, but also uses 2 1/2 times more reagent for the test, so your R-0871 bottle empties faster.

The CYA test is THE most difficult, by far, test of all, but it's very, very important. The MOST important part is to use the same technique every time. It takes a while to get the hang of it.