I’m sure some people plan to use Star Pieces during the GO Battle Day on Saturday, considering how profitable Battle Nights were in Season 8, but consider saving 2 for the Alolan Meowth spotlight hour on September 28 and some more for the October CD.
Alolan Meowth’s base stardust is 750, so unless you have no access to wild spawns, that spotlight hour could be the greatest stardust-farming opportunity we’ve had in a long time, even if it’s just an hour.
EDIT: As /u/humbertobx pointed out, it’s also a great opportunity to farm XL candy for Perrserker!
If you know how many pokemon you’re typically able to catch per minute (or seconds per catch), the table below should give you an idea of how much you can earn during that spotlight hour, and whether or not you want to spend Star Pieces. If you don’t know your number and want to figure it out, you could test yourself during the Skitty spotlight hour next week (similar catch-difficulty).
|cpm||spc||dust in 1h||x1.5|
dust per hour = seconds in an hour / seconds per catch * dust per catch
dust per hour = seconds in a minute * catch per minute * dust per catch
So even if you’re only able to catch 2 A-Meowths per minute, you can still earn as much as rank 20 players got from the GBL Season 8 rewards. Compare that to the Battle Day on Saturday, where the stardust from win rewards (the buttons you tap for stardust, items, encounters and rare candy) is quadrupled, but the end of set dust the same as usual (probably because of how profitable it was to insta-surrender every battle on previous Battle Nights).
With an active Star Piece at rank 20, a basic set with 1/5 wins will give 10005 dust, a basic set with 5/5 wins will give 22620 dust and a premium set with 3/5 wins will give 21255. Even if you’re somehow able to complete 21 basic sets with 1/5 wins in one hour with Star Pieces active, you’d still only get 210k dust. That’s barely better than catching 3 A-Meowths per minute.
The Duskull CD on October 9 will have a 3x catch stardust bonus. Multiply your dust number in the table above by 0.4 to figure out how much you can earn per hour when it’s 300 dust per catch (0.4 * 750 = 300). Even though Duskull only will give 300 dust per catch, most people can probably earn more dust in a couple of hours on that day than by going 1/5 in 21 sets on Saturday.
tldr: Don’t use Star Pieces on GO Battle Days unless you have more than enough for the A-Meowth Spotlight and the October CD (if you can participate in those events).