My friends and I did get a jump on a number this year -- Cleveland under 6. It's as low as 4.5 now. But the middle is hardly free. We had to pay -120 on the under 6. To play the over 4.5 right now costs a minimum of -130. So you are cutting into potential profit if your initial wager was correct and the "hedge" or "middle" bet loses, i.e. Cleveland wins 4 games or fewer.
I still have no idea why anyone would jump on the Dolphins over 7.5 in the early going. It's not a bargain, by any stretch, and the number figures to go down, if anything. There are already some spots using 7 with juice on the over. To tie up money for 7 months or more you need a theoretical advantage that matches or exceeds your other options for those funds, like simply putting it in a bank. I don't see how Miami over 7.5 offers that type of edge. I guess it sounds cute on message boards among fellow fans.
Here is a link that lists every team and the consensus over/under number and juice. You'll note the Dolphins are 7.5 under forty, which translates to -140 on under 7.5 and +120 on over 7.5. I hope the OP managed +120 or better. Otherwise he's already worse off than anyone who walked into a sportsbook today and decided to bet the Dolphins over.
At far right is the betting trend. It's extremely early, and low volume, but you'll note the trend on Miami is overwhelmingly under -- 95% to 5% over. That's what I mean, no reason to rush. If the widespread instinct is under and you are betting over, it doesn't make a fleck of sense to play now. Let them move it your way.