Looks like you are riding the blurple train , those light systems tend to require a bump in CalMag as grows go on.
Autos need very little in nute support , as most bagged mixes can carry an auto full term with top dressing or dry fertilizer scratched
in. Throwing bottles at it will cause more headaches than doing simple adjustments. You can run TAP water with FFOF.
Run the ph at or near FFOF soil ph ( 6.3 from bag ) , adjust 6.5 - 6.7 and leave it alone. FFOF initially can power an auto for weeks all by itself .... Some strains are sensitive to this hotter mix but will acclimate if you do not add more fertilizer on top.
That is one of the biggest mistakes with FFOF ... let it do the initial job in feed , then as plant begins to exhaust it a bit , add more soil or a simple dry fertilizer ( megacrop / Happy Frog fruit and flower / Dr. earth / Gaia or similar ) .... even a simple 4-4-4 npk. Most tap water will have cal / mag intact , RO and similar will NEED it replaced ( depending on filtering methods ).
I run FFOF this way and that way .... not much to overthink.
Also , take any hydro shop advice with a grain of salt ..... their job is to move product , whether for good intentions or not.