Calculating how many days of record time? Need help organizing drives

IReallyLikePizza2

Known around here
May 14, 2019
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Houston
Here is my current situation

LPR and "Back" clips I'm happy with. But around the front I don't want to reduce clip time to lower than 30 days

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I am adding another 4PM cam with Audio, so I could add it to "Front 2" but I don't want it to go less than 30 days, and I think it might. How do you guys calculate it out?

I have 20+ 8TB SAS disks spare, so I could add an 8TB disk, but what should I move to where? (Yeah, my signature says I have 4 x 12, but I stole some for something else...)