Okay, I haven't got much time to work on this tonight, so these are the kind of ranges that I'd like to see for the BSR ratings.
As stated earlier, CAP5 needs about 132 PT to avoid the 2HKO from Adamant LO Landorus-T's Earthquake after SR and a Sitrus Berry activation. I think it could potentially be harmful to let CAP5 become any physically bulkier than this, as it survives stuff like Scarf Salamence Outrage after SR, which it should really have no business taking. In addition, higher PT values could make it too easy for CAP5 to stall out certain physical attackers with Lum-Rest. I really don't want the PT to go any higher than this limit, but it can go lower if people want. So long as it isn't OHKO'd by stupid stuff like the aforementioned Politoed Ice Punch, then I'm fine.
CAP5 will absolutely need a ton of special bulk to take on Latios with any sort of reliability. 170 is the bare minimum for me, as this is enough to avoid the OHKO from Specs Draco Meteor or Sun HP Fire without hazards, assuming a fairly low HP stat with 252 EVs of investment. Any lower, and we start to compromise CAP5's ability to do its job effectively, as it would have to pick and choose the Latios sets that CAP5 has a chance to switch in to. 260 feels like a good maximum to me, as it allows CAP5 to avoid the 2HKO from Sun HP Fire after SR, assuming a fairly low HP stat with 252 EVs of investment and a double-Sitrus berry activation. However, it still allows offensive Ninetales to at least 2HKO it, as well as other Fire types such as Heatran, even if we go fully defensive. Any higher, and we start to compromise the ability for Fire types to come in and check it, as Lum-Rest will make it too difficult for them to break through. Overall, I think that this is a good range that encompasses everything that could fulfill our goals, depending on the other stats.
There is a reason for having such a ludicrously wide range. If we decide to go the slow wall route, then 60 PT allows us to have about a Base 70 Attack alongside Base 30 Speed. We aren't really able to go below that Attack stat without compromising CAP5's ability to use its Dark STABs, especially given their already low Base power. Meanwhile, I believe that 30 is the minimum Speed to go with, mostly for optics reasons. On the other hand, an upper limit of 200 allows us to go the "offensive response" route with Base 109 Attack and 111 Speed. The Speed is obviously for outspeeding Latios, while I don't think we could really go with a higher Attack in combination with that Speed without diverting focus away from CAP5's role as a supporter. As higher Base Speeds don't impact the PS that much, this should really be a hard upper limit. Obviously spreads that attempt to go the 160 / 70 route or similar to abuse this upper bound wouldn't be slated.
Since it's been decided that we don't want CAP5 to break through its counters and checks with Hidden Power (or any other Special move, for that matter), we can pretty much make Special Attack our dump stat. Thus, there's no real need for a lower limit other than for optics reasons. Meanwhile, an SS limit of 100 allows for high-Speed spreads to get away with Special Attack stats that don't screw over optics, such as 54 / 111.
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