>>532752282
Refer to this chart.
And use this tool:
https://gktools.ris.moe/en/memory-calculator

Basically the gist is that, each skill card has their own cost. And each memory has a minimum total cost of these skill cards depending on the rating of the run.
The lower the rarity of the skill card, the lower the cost.
So to efficiently farm your target deck, you'd have to pick the target skill cards for your memory, calculate their total cost and aim for that rating, and for the whole run, only pick low rarity silver skill cards so that they wouldn't be included in the final deck.
So yeah, lots of RNG.
There's also the green memory swap ticket you can get from the coin gacha and contest store but that shit is ass.