May I ask how long you are doing this and how you avoid assignment? Do you put your short leg deep OTM? I think assignment is inevitable in the event of black swan market crash.
On the other hand, If you have not taken any assignment from your short leg so far, what is the point to have a protective long leg ? Why not just go short naked and fully leveraged on the margin ?
Been doing it for a number of years. No assignment because the /ES options are European style (except for the options that expire this coming Friday) meaning they can only be exercised on expiration day. In any case I always take action on any position that's being challenged (well before the expiration day) in any case. As I said in a previous message I always close or roll a challenged position. But if, for example, there was a black swan event and the price blew through my position leaving one or both legs in the money then the worst that could happen is closing the position and taking a small loss (i.e. the difference between the stikes+commissions+fees.) Far more palatable to me then risking an opened ended (in the case of calls) loss.
Buying the long (protective) leg does two things, it defines the risk. I know, going in what the maximum profit and loss will be. Whereas undefined risk trades can theoretically produce an unlimited profit but also unlimited losses. And the undefined risk trade, i.e. selling calls and/or puts outright (e.g. a strangle or straddle) with no long position(s) for protection increases the capital requirements from about $450.00 for the 10 wide /ES vertical spread to about $9,700.00 for the one short position or strangle or straddle. I'd rather not commit that much capital to a single trade vice making a series of trades using less capital.
Finally, to answer your other question, my short leg is generally 16 Deltas OTM (1 SD) which gives me an 85% probability of expiring OTM. The long leg is usually 10 points farther OTM. This morning, for example I sold an /ES 3625/3615 Put spread for $1.25 and a 3730/3740 Call spread for $1.00. The Deltas for both short positions were 15 and 14 respectively. A little lopsided but it got me what wanted. This was for the Wednesday (Week 3, E3CZ20 ) options that expire the day after tomorrow.
In summation, no, I've never been assigned and, no, I don't do undefined risk trades. Nor intend to.
Best (and happy holidays)
BL