the irish one said:I had a similar problem a few months ago so heres what i did.
I watered the plant and weighed it straight after then weighed it the next day before watering and the difference in weight gave me a good idea of how much water was being used.
then i just placed the pot above a drip tray and set up a pump and dripper, i was only away for a week or so and the plant was still under 18/6 so things worked out fine.
Your situation strikes me as being a little more awkward seems as your plant has just been fliped into flowering and is going to stretch.
You will want to make sure that your plant does'nt stretch up and get burnt by the lights.
hope it goes well for you stay safe.