If you want the POI to come up 4" at 15 yards, (assuming you're happy with "driving the dot") you'll need a front that's about .040" shorter than your rear sight. Heinie makes a stock tritium set that'll get you really close. And Dawson makes a combo that'll get you there too. I use the latter in a really similar height relationship on my pistols.
FWIW, Sigs typically don't come out of the box (or use stock sights) and print where I like them to.
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