You didn't ruin your batch, what we're saying is that you run a higher risk of a stuck fermentation or incomplete attenuation by racking early. It's safer to wait and be sure your at FG before transferring to secondary, because once you rack it most of the yeast is left behind. Just something to consider for the next batch, that's all.
Regarding testing the same sample (this is called a satellite fermentation), I don't believe that the tiny sample a homebrewer uses will ferment at the same rate as the whole batch. It probably works on a large commercial scale, where the satellite fermentation is bigger than your whole batch.
I don't like to pull too many samples either. After I take my initial gravity reading, I wait at least 10 days to take another. Usually that sample shows that it has reached its final gravity and I'm ready to rack into secondary. To me there's no point in checking the gravity before that because I'm not going to take any action prior to 10 days, and there's no need for any samples after that because I know it's finished. IMO, any other samples are unnecessary unless something is not as expected.
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Primary 2: Edwort's Robust Porter
Secondary 1: LW Pale Ale
Secondary 1: Blackened Soul RIS
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