Like everyone else in isolation, I've been playing a lot of Animal Crossing: New Horizons. In order to jump to the top of the socioeconomic ladder, I'm taking part in the Stalk Market. If you didn't guess from context, the Stalk Market is like the stock market so you want to buy as low as you can, and sell as high as you can. If you're playing with friends, this is a lot more effective.
I made this calculator to make it easy to calculate risk and profits. If you bought a lot of turnips and you want to hedge your bets, a great way to do this is to sell a percentage of your investment to break even, or at least get close. You can use this to calculate what price to do that at, and how much of your stock to clear when you do. If you're being shown a price and you want to know how much of your current stock it'd deplete to break even, you can choose Best Turnip Price to enter a price and see how good it looks. It's pretty simple, but I find myself doing it over and over again so it's nice to have. If you have any feature requests for it, you can do that by contacting me.
Note: Turnips are sold in multiples of 10, so each stack is worth 100. Calculations assume that everything is done in the number you have, rather than the number in your inventory. i.e. I have 59 stacks of 10, so I would enter 5900 in the field below.