Using basic calculations from MFG and other websites will always get you into the ballpark but if you really need to know you need to run it through the manufacturer. If your local dealer wont provide you with the Factory's Calculations get a different dealer.

There are a lot of different factors which allow one model of truck to "Maintain" capacity better then another. For example a 3 wheel electric sit down truck will typically maintain capacity for attachments, and taller masts better than a 4 Wheel electric truck. The 3 wheel truck normally has a wider stance, and lower center of gravity because the battery is closer to the floor allowing it to maintain capacity better than its 4 wheel counterpart. The the online and available formulas dont necessarily take these differences into account and therefore should only be used to ESTIMATE.