With coco coir base? That's 'soilless' so every irrigation... and every irrigation should have 10% runoff or more to avoid buildup.
in a soilless context, you maintain a consistent, controlled level of nutrients around the roots. The runoff ensure you don't ge buildup -- even if the resulting equilibrium is not exactly what your formula is... it's still directly controlled by your formula, which is all that matters.
Calculated elemental ppm is more accurate than an EC pet reading that is just a conversion of electrical conductivity. Don't compare the following numbers to EC readings / conversions.
N 120-130
P 40-60
K 180-200
Ca 100+
Mg 75ish
S 100ish.
This would be a low concentration and a good ballpark to start. I use somethign similar on seedlings and never have an issue. So, using from the first irrigation is no problem, though you can rely on the seed to power itself the first ~10 days or so. sometimes i wait for the 2nd fertigation, but seem to get better results starting day 1.
I use promix, which comes with an inidtial charge that is extremely similar, except for much lower potassium. So, in my context whether i fertilize with the first irrigation or not, it gets roughly the same thing from the get-go.
from here you can simply observe and adjust... this basic formula, with your adjustments for local variables, will work well on 99% of marijuana plants. It's not my formula. IT's a formula used by several professional Ag companies for 'hydro' nute setups... hydro and soilless in regard to fertilization is essentially the same thing.
there are websites and apps that can calculate and tabulate your elemmental ppm from gauranteed analysis lables and dosage. The better ones can even tell you the dosage to hit certain ppm targets. This is how you can use just about any brand of products to get a similar ratio/concentration. if it's a soilless/hydro fertilizer, it's 100% plant available and 100% soluble, so it'll translate across brands without much concern.
jacks 321, megacrop, crop salts, sothern ag, masterblend et al all have a ~3 product setup that essentially equates to the above ratio/concentration, but that doesn't mean you can't get there with other products. Brand is mostly irrelevant when dealing with ubiquitous ingredients for soilless/hydro products.
check out cocoforcannabis.com. their articles and guides are the best, bar none. The least amount of nonsense science out there. compare their guides to others to see what you should ignore, lol. All sorts of conjecture and unproven hypothesis of people that have no background education or experience on the matter. It's easy to avoid the really retarded stuff like burning your plant or icing the roots, but there's plenty of stuff that sounds right but makes no sense...
e.g. anyone that thinks flushing changes the mineral content of buds - developing or not. Simply no biological process exists in the plant where it can change the mineral content of finished buds after the fact. There is no excretion system for such things. Toxicities and eficiencies may impact the end result of flowers, but they are still built according to DNA instructions. Molecules can't just be built whimisically with more Ca.. the molecule wouldn't function properly if it were constructed differently. The whole plant would fail if this were the case.