Potential is for the player, not for individual skills. Each skill has a different effect on the potential it uses, depending on the overall skillset. If your player is a Center, raising Inside Shot 1 level eats a good amount of potential. If your player is a PG however, raising Inside shot 1 level barely uses any potential at all.
Also, the potential in Buzzer Manager is not up to date. There have been changes to Shotblocking and Jump Range which are not reflected in the tool; they use less potential in reality than in the tool.
The value of potential on a player depends on wether it will be a limiting factor, and that depends on your skill in training, your trainer/facilities and his starting skills. Usually, a new player doesn't have a lot of skills/assets for training, so a player with high potential doesn't mean much since a new player probably won't be able to train that player to it's cap. The better your skills/training assets, the more important the potential becomes. Also, your goal with the players matter a lot; if you want to train a balanced SF you need less potential than a traditional Center.
Seeing that you're into training, inquisitive enough to increase your training skills and willingness to invest in training assets, I'd say you can use higher potential players. I'd go for 7 minimum, 8 is fine and 9 is nice to have.
Also, to compare 18 and 19 year old draftees, consider that the 1st year of training can yield 10-18 skill points (depending on your training assets and which skills you train). So relatively speaking, your 18-yo player with 47 has more TSP than your 19-yo player with 58 TSP. He also has higher potential. However, you can get an 18-yo 8 potential player with 50+ TSP for cheap on the market, which makes for an even better trainee. They won't be home grown (and possibly not even from your country), so that means less merchandising income.
Enough food for thoughts I assume :)