As I mentioned, you cannot just do it randomly. It won't work. You need to enter at the right time and the right prices. And when you compare ATM prices, you ignore gamma gains, which are responsible to significant chunk of the gains.
As I wrote, include some with strikes that were near the money a week and two weeks ago. Then graph the two week straddles (intraday, not closing prices). They were ATM back then and by looking at their performance over the past week or two so if there are any gamma gains, they will be evident if underlying price has moved.
If you want to prove the efficacy of your system and silence naysayers, after you alert your subscribers to a position, post the trade details here after a reasonable delay. For example, we bought an XYZ straddle at $5.40 1/2 an hour ago and then readers can evaluate in almost real time going forward. Without that, it's just your word and not many will accept that as proof positive in a world full of internet scammers. And if you picks do succeed, I bet that you will get more subscribers as well.
I do, however, question the ability of your clients to make money based on your subscription alerts (described somewhere before).