Okay, so as not to veer too far from the OP, let's examine that probability modeler on Larry McMillan's site:
https://www.optionstrategist.com/calculators/probability
Is there anything there that is different from an inverted "Expected Move" calculation? No.
It's got a market price, a strike price, a DTE space, and a vol space. So, no different than an Expected Move from ye ol' BSM framework: you've got delta, theta, vega covered (and poor ol' rho is still left dangling in the wind, as pretty much everybody has done...).
Any need for (
or use of) Monte Carlo in here? Nope. In fact, there *can't*be* (despite the verbage above the inputs, "Calculate stock market probabilities with this easy to use Monte Carlo simulation program"), because there is no mention of any number (fixed or user-input) of trials.

"Hello!"
But the OP (I surmise) wants something broader than just options anyway -- maybe shorts & longs and then puts & calls to support those underlying positions? Regardless, the same inputs for the "calculator" fill the bill: it's still BSM-derived Expected Move time.
As for a probability distribution? Well of course, you still have one at work in that volatility number you have for the spreadsheet inputs: Gaussian Normal in a returns world, logNormal in a market price world. [In my own experience, I mapped out strikes on either side of the market, coded in the Expected Move, as well as stoplight colors at the strikes from |.25| down to |.15| red, from |.15| to |.10| yellow, and from |10| to |.05| green. And outside of all that? a stoplight color round-up based on a Gaussian/Normal, just to contrast with the IV-driven deltas.]
So, how to 'come up with said distribution'? I dunno! I coded Normal because it was symmetric, and allowed me a precise framework to view skew. Over longer term analyses (6-24 months), a logNormal might be more supportable, theory-wise. Pareto -- I've seen that sort of shape *way* too often to ignore, but in shorter-term option spreads, it's simply not been needed. My favorite to *think* about is Weibull -- with four parameters -- with which you can mimic Gauss, Pareto, and logNormal.
At any rate, if this were about *portfolio* management, I would insure that each underlying had
its own supported distribution -- maybe based on its short-term IV?? -- and use those proffered distributions as lenses on the individual underlyings and their outlook. I mean, even as an index trader, I would never dream of using S&P VIX on the RUT, right? Ewwwww.
Thems me thoughts.
