How to calculate disk space required by Prometheus v2.2?
I think your math may be off.  If you use the ~18600 number you found I get a very different result from 68TiB:

2,592,000 (seconds) * 18600 (samples/second) * 1.3 (bytes/sample) = 62,674,560,000 (bytes).  This is ~62.675 Gigabytes (divide the digital storage value by 1e+9
) and I'm guessing a more reasonable number for your infrastructure.