I find the calculator to be more of a backup than the final word on how much you take, because if you enter the wrong numbers (as in this case presumably) you can end up being WAY off. Still, it's good to have some help with the numbers, and I don't think the calc's are linked enough.
Online (bottom of page)
http://www.dextroverse.org/whatis.html#polistirex
DXM Dosage Calculator by sonofagun Download
http://www.dextroverse.org/images/dexcalc.exe
I think what happened is that you accidentally entered your weight in kilos (kg), rather than pounds, therefore telling the calc you weigh over twice as much as you actually do. Make sure you check US pounds/Lbs depending on the calc. Or better yet,
convert your weight to Kilos, as that makes it much easier to deal with the mg per Kg system everybody uses. Just divide your weight by 2.2. Thus my 137 Lbs becomes 62 Kg, your 250 Lbs is 114 Kg, and your standard dose is about 6 Mg/Kg.
What you're talking about here, at 250 Lbs, is jumping from about 10.5 to 12.3 Mg/Kg, both of which fall well within the range of the 3rd plateau, so you should see only a moderate difference. But we're talking 3rd here. Many people, upon encountering third, are so blown away by its effects that they chalk it up to being a 4th plateau. But there's a distinct difference, it's not just a matter of opinion. 4th Plateau trips are INTENSE, basically completing the virtual paralysis 3rd starts; when you came down you probably wouldn't even remember the majority of the trip, and I doubt you would be this eager to repeat the experience this soon. Anyway, if you were aiming for a 4th, which you should seriously think about first, your projected threshold would be 1680^.
1st Plateau: 1.5-2.5 mg/kg
2nd Plateau: 2.5-7.5 mg/kg
3rd Plateau: 7.5-15 mg/kg
4th Plateau: +15 mg/kg
20 mg/kg: Serious OD Risk