Just to throw this out there, I've read that lumen output also has to do with distance, so if you are assuming a certain number of watts or lumens per foot, you also need to take into account the distance of the lights at the time... a 250w light might only be 100w by the time its light gets to the plants if its too far... and visca versa.

There is also a coorolation with wattages, I noted my 250W actually pulls about 450Watts and my 400W HPS pulls about 700 Actual watts, so keep this in mind when planning breakers and fuses etc...