You *could* do a calculation based on Ohm's law and a nominal resistance for the bulb (the latter calculated from the formula for power, using the nominal wattage of the bulb), but it would be meaningless because the resistance of the bulb varies greatly with current. If your unit is not shorting, the bulb will be working well under its rated wattage, and the unit under test will get near full line voltage, no matter what wattage bulb you use in a DBT (a bulb designed for full AC line voltage, that is).
The bulb only dissipates its nominal wattage when full AC line voltage is across it. When a unit is wired in series with a DBT, each will get a fraction of the line voltage, the split depending on their relative resistances. If the unit under test is not shorting, it will have a large share of the resistance in the circuit and therefore will be limiting the current flowing in the circuit, and the voltage drop across the bulb will be a very small share of the line voltage. In this circumstance, the resistance of the bulb will be low. Only if the unit has a short will the full AC line voltage be across the bulb. In that case, the bulb's resistance will ramp up to the value calculated from its watt rating, and it will limit the current (and drop the voltage) passed onto the unit. This little dance with variable resistance, variable voltage drop, and variable current limiting provided by the bulb is the whole idea behind a DBT.
So get a variac (the ones that handle 5A or 500VA aren't that expensive and should be enough for all but the highest power amps), or get a 600W rotary light dimmer (one that works for incandescent lamps) and wire it between the wall outlet and your DBT. They are only $10 or so home improvement places, plus the cost a wall box and wire/plug. I've not tried that for this purpose, but it should work.