Hey Boss, you have two sets of leafs,It's time to add 1/4 the strength nutr.I use GH 3-part nutrs.They have a feeding chart at hydroponics.net go to the bottom of the page and click on learning center.there's a ton of infor and the weekly feeding charts and more.
To calibrate your pen you will have to buy the calibration soluition,4.01 & 7.01.You cant use your nutr solution to calibrate with.On the pen there's two trim screws,one for the low ph 4.01 and one for the high 7.01.You put the pen in the 4.01 solution and turn the screw and the same with the 7.01 sloution.Now it's calibrated.You will do this about one time a month ,just so the pen will give you the right PH reading.To have a early and late crop just start one and 5 weeks later start the second. Best of luck to you. PEACE