By Maxx Chatsko Well, it depends on how you calculate the average return. How many times have you read or heard that the S&P 500 returns 10% per year? It’s an easy number to toss around. However, if that number was wrong, then it could have dire consequences for your retirement planning. Unfortunately, the S&P 500 doesn’t return anywhere close to 10% per year. The actual average return — after adjusting for inflation, reinvesting dividends, and assuming you pay no taxes– is almost half that. How is the oft-quoted figure promising 10% average annual returns so far off? It all comes down to th…