Download a light measurement app on your phone. (For example, Photone) With that, you can measure the incoming light amount at the plant's tip very well and then adjust the lamp accordingly. You can't really do much with "700 Watts" as a blanket statement; the type of lamp, manufacturer, etc., have a big impact on performance. Basically, they are very sensitive in the first few days, especially with such long (18h) uninterrupted artificial sunshine. Test it out carefully, ideally write down the values (you'll need them again for the next grow), and keep a close eye on the plant. As soon as it gets too little light, it starts to stretch. Then you should definitely give it a bit more light. Too much light can be recognized by the lighter or yellowing shoots at the point closest to the light.