I would not play the upbeat as a downstroke. My suggestion is to get your metronome out and set it for 4/4 time, with eighth notes. To get the reggae feel, practice the upstrokes on the "and" of each measure:
1 AND 2 AND 3 AND 4 AND
If memory serves me, that upstroke is a quick flick of sorts, not a full strum where you are aiming to apply your stroke equally across all six strings. Hope that helps.
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