It's dynamic. The curve. I can dumb it down but it would require me to spend an hour on it. I am too busy/lazy to do it.
You deserve my thanks for asking all the right questions I was unable to ask. Originally I was interested in 121 equal distance because I wanted to try delta neutral straddle/strangle but did not like the unlimited risk of writing those. Flies gave me tail protection. The 132 is unequal distances. For example: 1490/1505/1535 calls, the wings are 15 & 30, upside loss is high (the 2 of 132 is higher strike 1535). I am way behind you in understanding and still don't know why it is preferred by the professional. There are so many moving parts I think I have to write a VBA Excel to get further into the details. Trying to do it analytically is beyond my limited math capability.
Well, you've got the structure wrong. A 132 call fly would be a 1490/1505/1512.5 (permitting), classically. There is no rule that you can't go symmetrical on strikes. The rationale for the 2/1 strike width is that the debit req = risk beyond both wings, that's it. The debit is the max risk in the fly.
Ah my bad. I quoted your 1490/1505/1535P instead of C. Yes, thank you for correcting me. Now it makes sense. Will run through some structures and see the effects and set ups.
There are no limitations, only w.r.t. symmetry on PNL outside the wings. Stick to 132s at 2/1 strike width if that matters to you, but I often trade symmetrical (and beyond (again asym, approaching 1/2 widths) on strikes to achieve a credit on the fly.
Well, this made me realize I was chasing the wrong profit diagram. I thought it needs to be something like this: While it has to be like this:
But this is such a narrow profit range. How can I guess where the stock will end up? And also how can I set the fly such as it's delta neutral where I want it? The non active fly in the simulation is quite on the other side, but delta is pretty much the same as for the active one: -2.75 vs -2.39.