On it's face value assuming you could pin price at expiration the single calendar would obviously be the more efficient ROC scenario, but as is well known things rarely go as planned. I've seen a fair amount of comparisons of the single versus double spreads and am familiar with the tradeoffs of each. That being said I haven't seen hardly anything on the topic of whether it is more practical to start with a single and convert it to a double if needed or just start with a double from the outset. I would be interested in hearing the thoughts of those who are familiar with this type of trade and why you prefer one method over another.
You will probably find a lot of discussion about the terrible risk reward of a double.Good luck on getting input though. My preferred adjustment is to hedge with underlying until i can make a favorable exit.
You don't trust your ability to forecast an underlying's direction and are afraid of losing money. 1) Teach yourself to predict where price will be in some timeframe more than 50% of the time. 2) If you really like calendars, position OTM calendars where you think price will go.
I'm assuming you're trying initially to express a market neutral view. If so, there's no reason not to go with the ATM calendar and add another position if your view of neutrality changes. The subsequent trade might be another calendar, but not necessarily. Why obligate yourself to adding another calendar simply because you started with that position?