When you say something is up 300%, you mean the price has tripled. You are referring to the AMOUNT OF CHANGE in the original price, not the AMOUNT OF INCREASE. What you are saying is it is now 300% of its original price. 100% + 100% + 100%.
So if an SP has gone up from $1 to $1.50, then it is up 150%. You could say the SP has INCREASED 50%, but that is a different thing.
If you don't see it, try it on your calculator. 10 x 150 %button. The answer is 15. So 15 is 150% of 10. And if 10 goes to 15, it is up 150%.
English language allows for a lot of flexibility, so things can be mis-interpreted. The only real rule is usage. I know some will still disagree. All I can say is look at your calculator again. There is a % button to give you percentages. There isn't a % button that says "only add the percentage increase". We are not talking about BY HOW MUCH something has increased.
Last example. You have a pool with 100 litres of water in it. You add 200 litres of water. The pool now has 300 litres of water. The total volume of water is 300% of the original, not 200%.
To work out percentages with calculator. Take current amount, divide by original amount, press % button.
