When the resistance is fixed such as is the case here with heating elements, you don't have to calculate its value. All you need are the 2 different voltages involved, and the known number of rated Watts.
We can determine the Amps at 240 Volts by taking 10,000/240 = 41.67
Next, set up a proportion equation thus:
240 Volts = 41.67 Amps
208 Volts = (x) Amps
Solve for (x)
208 times 41.67 = 240(x)
8667.36/240 = (x)
36.11 = (x)
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A loose rule of thumb is the watts will be 75% value at 208 Volts, VS 240 Volts, as shown in other calculations in this thread.
The reduction of the Amps will be proportional to the change in Volts, in this case 86.7%