If you want Stonington Gray but you're buying Dulux — for trade availability, mixing convenience, or just because your decorator stocks it — go for Dulux Slow Living. It lands at ΔE 2.1 from the original, which is well inside the "very close" band (anything under 2.5 is genuinely hard to tell apart on a wall). Its LRV is 52.6, so you keep that same airy, mid-light feel that makes Stonington Gray so useful.
Running a whisker behind is Dulux Silver Shores at ΔE 2.2, LRV 50.5. Honestly, the gap between the two is negligible — both sit a hair lighter and within touching distance of the original. If your local merchant has one and not the other, don't agonise over it.
The "but what about" here is the classic Stonington Gray trait: it's a grey that leans cool with a quiet blue undertone, and that undertone shifts noticeably with light. In a north-facing room it'll read cooler and more blue; in warm afternoon light it softens towards a true grey. Both Dulux matches inherit that behaviour, so the colour will move on you exactly the way the Benjamin Moore original does. That's a feature, not a fault — but it means you must test before committing.
Paint a decent-sized patch (A3 minimum, or use a sample board you can move around the room) and look at it morning, midday and under your evening lighting. Cool greys like this can tip sterile or even faintly lilac in poor north light, so check it against your flooring and trim before you buy the full quantity.
One practical note: a colour match is just the colour. The finish, opacity and feel of Dulux won't be identical to Benjamin Moore's paint film, so if you're patching into existing Benjamin Moore walls you're better off staying with the original. For a fresh job, either Dulux match will do you proudly.