I have done capacity testing for two different forklift OEMs, one with off-road telehandlers and one with 1-8t strait mast industrial. The capacity of the truck is determined by tipping it in 5 different scenarios to see if it will go beyond X degrees before it tips. (Note: lifting 1 wheel is not considered "tipped") Repeat for each attachment combination.
In the case of telehandlers several weights must be tested at several different extensions to create the load chart. Operator weight is either 0lbs or 200lbs, whichever is the worst scenario. The fuel tank level is also dictated by the worst case scenario. And tire make/model, size and inflation pressure have a huge influence.
In the case of counterbalance trucks there can be hundreds of different mast, carriage, fork, engine, fuel type, tire and counterweight combinations.
I can tell you from experience that it is very difficult to calculate the actual tipping points. Telehandlers have few options so they are usually tested. For industrial trucks the OEMs have built their own database with past experience that has helped them develop fudge factors for the different options. Then they only have to test the major changes from one model to the next, put that in their model, and they can calculate the hundreds of permutations.
The big concern that OSHA has is nobody but the OEM is going to know or understand what all goes into these stability calculations. And when Joe Bob modifies his forklift and kills himself, the widow and her three little kids go after the big bad forklift company. We had one case were the guy admitted to drinking a pint of vodka and a 6 pack of beer for lunch...and he still won!?!
As unclesyd was saying, OSHA doesn't mess around with this. And you can see why the forklift companies will not approve your homebrew design, unless you want to pay some money to have the design reviewed and possibly tested.
ISZ