I don’t think you should approach wi th a nutrient strength goal in mind. Instead, you should view each plant as it’s own genetic makeup. When I grew DWC, I would have two plants from the same pack, and they wanted different nute strength and different light heights. It’s crazy how unique each plant is.
Having the extra reservoir with a top off valve is a great idea. It saves on mixing effort (why I eventually left DWC) and could self sustain for a bit if the ph and ppm’s are steady. I would fill the top off reservoir with nutes about 30-50% of the Target for the main pot. You will be amazed how things will climb otherwise.
Also, during my first DWC grow was in my garage in the Sacramento area. Brutal! I got a water chiller, and set it at 66 degrees, and never had a problem even though the cabinet would regularly hot triple digits daily. It’s not ideal, but root temps are the key. you keep that down, and the roots healthy and happy, and the plant can handle a lot.
Good luck, bro! And have fun.