One percent of the account is a common starting point, not a law. On money you need for rent, one percent is already too much. On a practice account you can use the same percent so the habit is right, even when the dollars are pretend.
The formula is short. Risk in dollars = account × risk percent. Risk per share = the difference between entry and stop. Shares = risk in dollars divided by risk per share, rounded down. Always down.
What goes wrong is choosing the share count first, because it feels like a real trade, and drawing the stop afterward so the math works. Then the stop is an excuse, not the level where the idea is wrong.
Add the spread. If the stock is trading 4.10–4.18, your real entry may be 4.18, not the last print at 4.10. Use the worse price. If you do not, the calculator is a story.
When the day's loss hits the ceiling, you stop. Not after one more. The ceiling should be smaller than the amount that makes you angry, because angry traders trade worse, and the next loss is bigger than the formula.
Try the calculator under Tools. Enter numbers you actually have, not numbers you wish you had. If the share count comes out embarrassingly small, that is a good answer. It means the distance to the stop is honest.