I've used Big Bud since they invented it. I like to feed a half dose of it a week before flipping to flower then give the other half at the flip. Then a regular dose the watering after that. And every 2nd watering until the end of week 5 for a strain that finishes in 9 weeks. Some times you don't know for sure when a plant will finish until you've grown it out at least once. Plants from the same strain grown from seed may finish at different times too but clones should be the same.
Most P/K flower boosters are higher in P than K compared to BB as it's 0-15-35 with 10%S, 20 amino acids, ascorbic acid, (Vitamin C), and citric acid. A while back I won some Greenleaf nutrients in a contest here. I got 2kg of the MegaCrop base nutes and 1kg each of the Green Terra Big Up p/k which is 0-52-34, Rock Steady which is K alone at 0-0-62 and the Carbo which is pure dextrose sugar. Carbs are used to feed the micro-herd, (Good bacteria and fungi more useful in organic growing), so not that important when using hydro nutes in a soilless media like the HP. You never want to add carbs to something like DWC.
I've been reading lately that higher P is more important in early flowering then demand drops off after the stretch and the plant wants more K so I may use the GreenLeaf p/k early on and switch to the Big Bud after the stretch.
Something else I do after the stretch is to feed the plants Lucas Formula style. That just means to not use the Grow and use twice as much Bloom as Micro. Basically a 0-1-2 ratio of the 3 parts which are referred to as GMB, (Grow, Micro, Bloom). This is too reduce the amount of N in later flowering as too much N inhibits flower growth and if the plant needs more it can steal it from the older fan leaves as it's a mobile nutrient. I like to see my fan leaves begin to yellow the last few weeks as long as it's not too fast and mainly just older leaves.
Extra S is good in later flowering to promote resin development and that's a good time to add more Epsom Salts. S is the 'secret ingredient' in bloom boosters and it's hard to overdose with it. A tsp/gal is good enough and the drug store stuff is fine and cheap.
Most of the other supplements AN and everybody else sells is too expensive and not worth buying for maybe a 10% larger yield. A couple years ago I got a 500ml bottle of the Bud Factor X for free and after trying it would not go out and buy it. Almost $100/L and I got my 3 gallons of the 3-part for $124.
Just the 3-part alone will grow decent buds but I would boost up the amount of Bloom used when you flip to flower if not using any kind of bloom booster. Feed at a ratio of 2-2-3 maybe just before the flip and until the end of week 4 then go Lucas style to the end would work well.
The best aspect of a 3-part nute is as you learn more about the different stages of plant growth you can easily tailor the amount of each you give the plants to feed them better tho just giving them a ratio of 1-1-1 all the way will work too.