Strike Last says 'before any other modifiers are applied' so I think the best way is to work out what your end modifier will be, then apply it to I1.
So if I'm charging more than 3" to the front and have a hex on me with -2I, then I would have a modifier of +1 overall, which I would add to my I1 from strikes last which it was reduced to first before the others, for a total of I2 in this case
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