Welcome!

It is exactly how it is described in the image you posted.

The minimum for Altitude members is 250, based on a 100% accumulation rate.

So for a flight that is less than 250 miles in actual length, they round up to 250, and then apply the accumulation rate. So a Tango domestic flight is 25% of 250 , or 63. Business Class is 150% of 250, or 375.