If there is one I have never came across it and would think such a calc isn't possible due to difference in output from the various manufacturers of lamps and ballasts especially in an OD state. With this being said I assume the printed BTL output can be used to determine overall wattage if lamps are driven 2x their rating within reason. I say this because I have used a 2 x 65watts CF ballast to pump 130watts into a single 32watts 48" lamp and the output was a tad brighter than my 58watts driven lamp and definitely not even close to 80watts. A similar feat was tried with 65watts to a 18watts lamp and it looked as bright as 32~40watts. There wasn't a major difference as I expected so I stick to the 2x range of a lamps spec and find a ballast thats gives very close to or double the lamp spec output.
example: I have a 4 x 32watt 48" HD Phillips 6500K DIY setup driven by a Ballastwise ballast with a BTL of 4 x 58watts so I calculate wattage to be 232watts. 4 x 65watts would be the the most I would pump into this configuration and would calculate this to be 260watts.
The 穀精 Junkie