I'd go with 3 for autos, the plants never get that big, and you're pouring nutrients through every day, I find the 5s a bit heavy to move around).
I just use the mix levels suggested on the back of the WESCO packet and forget about PPM/T.D.S. I did use a T.D.S. pen a few times, think it was 300-500.
I chucked in 1/2 cap of cal-mag once a week as well, but not convinced it did anything worthwhile (the WESCO 30-20-10 contains trace elements).
I did a couple of autos, but didn't find the crops worthwhile.