There is no rule of thumb and it's not a given that anyone will look at the EFA-10 anyway. Just don't do a straw a purchase. A straw purchase is not selling with the intent to resell, which is what many think it is. The ATF has fundamentally centered their definition and investigations of straw purchases to a scenario where a purchasing agent acquires a firearm for someone who they know cannot legally possess one, or would not pass a background check. This knowledge must be apparent and substantiated between two parties, and some level of financial compensation is involved. If it did not involve the receiver being prohibited, then gift buying would be considered a straw purchase.
If you know the person you are selling to is a prohibited person, and you bought the item with the predetermined intent to "sell" to that person, it doesn't matter how many days have passed at the state level as you've committed a federal crime.
If you buy something and the next day someone says, "I'd really like to buy that from you," that's not a straw purchase - even if you thought, at some point, that person would want the item.
Also, in Massachusetts, all private, legal sales must occur between two parties with active, valid LTCs. It's presumed that someone with a valid LTC isn't a prohibited person. It's very difficult to conduct a straw purchase in this state as long as both parties have active, valid LTCs.
In summary: as long as you're not doing what you know you shouldn't, all is well. I know people who bought something and sold it to a friend the same day (they did not like the item).
To be extra safe, though not necessary: just go through the FFL to conduct the transaction between you and the buyer. If this can't be done, I would seriously reconsider the transaction.
