You won't find an online investment calculator that agrees with you because it's dead wrong.
Do this -- open up an account. Put $40,000 in it. Note your margin buying power. Short $40,000 or something. Watch a bunch of margin buying power disappear! Magic!
The $40,000 in cash is listed as "margin reserve" and it will show, in this example, -10,000 shares of JBII and it's listed as a debt, despite the cash. You can't then withdraw the $80,000 cash and go home lol. Doesn't work that way. Your loot is tied up. Your risk is there. Your investment is there. Hence the profit is only 75% and not the mythical number you posted.
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