Let me see if I can make it easy for him. When you have doubled your money you are up 100%, not 200%. It's your original investment plus the 100% increase that gives you the double.
So when you're up 300% the value of your money will be your original investment plus the 3x you've made on that investment.
.70 + (3 x .70) = 2.80
Got it?
Have you figured out the reason why you can never be up 300% when you've shorted a stock, only a maximum of 100% (your original investment - assuming the share price goes to zero)?