Supz Dave. I think I have an answer for your question so lemme try to word it out

Basically, every strain of plant will react differently to dosages of nutes. Some like it hot and heavy while others enjoy the softer side of life. The first signs of TOO MUCH NUTES include yellowing tips/leaf curl/etc. The first signs I usually notice when you have TOO LITTLE NUTES is droopiness of leaves/brown spots on foliage due to nitrogen deficiency/etc. Like I said before though, EVERY strain reacts differently. Now to the "scientific" aspect of the above. When using a PPM meter (golden btw) take the reading when you do a rez change and jot it down somewhere or remember it. I have 200 ppmz of dissolved solids in my tap water SO THEREFORE when I mix nutes I take the number I get and subtract 200 to get my base nute ppmz. With that being said, your plant will drink like a beast during lights on, and grow and shit-ton during lights off. Basically, at least 24 hours later check your water again ( I stir up my reservoir if possible to make sure everything is mixed up) and see if the number dropped or stayed the same. If the number dropped then obviously your plant is eating and the only thing you wouldnt know is how long it would take to eat the whole bucket because some days they eat more than others. EVERY TIME YOUR PLANT EATS NUTRIENTS YOUR PH WILL GO UP. IF YOUR PH GOES DOWN AND YOUR PPMZ STAY THE SAME OR CLOSE TO THEN YOU MAY HAVE TOO MUCH NUTES AND NEED TO DILUTE IT WITH PH'D WATER ONLY. Sorry for the caps, but I just wanted to make sure you saw it lulz. When nutez are in your water it makes the solution slightly acidic, which is the way plants like it, and therefore when they eat and those nutes disappear it brings your water CLOSER to alkaline or NEUTRAL. The way I tell how my girlz like my feeding schedule is to watch my PH levels in accordance with PPM levels. If your PH goes up GRADUALLY and your PPMz drop gradually then that's a good mix and you should stick with it. No sense in adding more nutrients than your plant can eat because 1) That shit's not cheap 2) LESS IS MORE IN DWC 3) Who the hell wants to waste perfectly good nutes?? lulz So as an answer to your ?, it's a combo of BOTH watching your plant and monitoring the numbers. You dont have to be uber-jumpy about anything and I caution you not to do so. Most of the difficulty in growing is just knowing when to leave em the hell alone haha. Pretty hard to do though when you have all that pretty green and the aromas mixed in with it tho eh? Hope this helps you man. I'm always around so if I can explain any better just holla
Oh yea your girl grows like my BL pheno

Stretchy and solid limbs, which later yielded MASSIVE colas! I tied mine down sideways just like you did and damn was it gorgeous 8 weeks later...good job man and keep her comin!!