Beafush, I would like to suggest to you to try the method you have done before to find the chunks that are relevant to your ore search, in this case: tin. Instead of digging straight down or in a staircase, I usually try and find some of the caves (I like the vertical ones) and use ladders to descend until you are in a larger space and use the Node search on walls, cave ceilings, and floors as you go down to help you locate a space that has the ore you're after and then you can proceed with the tutorial videos that you've mentioned.
I think the struggle that you're having is the disconnect between finding a chunk that is likely to have it (remember that those numbers that it gives you are not percentages (parts per 100), but instead per mille (per 1000)) and finding a starting point for the node search.
Alternatively, when I was first learning the propick, I started a clean world and went straight for creative mode, gave myself a lanturn and propick and searched until I got to an area that had what I was after and used it as practice without the baddies. (when you use the pro-pick in creative, you only need one rock instead of the 3 for the density search)