Okay, so you now know what's in development, I'll make it into a nice calculator for you. Easy. ...
In the meantime, you can use the link to read how to trade on Bitmex.
Warning: don't ever keep big % of your funds on Bitmex, I don't trust them. They have volume, but who knows what they do with it.
Use the referral link given in the tool to register a new account that has 10% on their fees for 6 months, so you pay them less. This way you use their marketing scheme that allows for trading discount.
Read around and see the logic of the calculator, the more you manually understand it, the more it will be worth to you.
Give me about a week, as I want to do it nicely and find someone capable to do it right.