Calculating alternative minimum taxable income (AMTI)

For the capital loss carryover worksheet/Line E - AMT, I'm told to input my Alternative minimum taxable income (Form 6251, Line 4).

However, I don't have Form 6251 from last year. What should I put in here?

The help article says most people put in their "regular" taxable income Form 1040, line 10 (which also seems strange because line 10 is adjustments to income from Schedule 1, Line 26).

But more pertinently, for myself the help article says AMTI maybe different than my "regular" taxable income even if I didn't have to pay AMT because I supported multiple dependents.

So how do I calculate my AMTI if I think my AMTI is probably different than my "regular" taxable income?

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