Computers use checksums to ensure that data was not corrupted in transmission. Now your brain can use a checksum for your mental math, with a few easy techniques.
Concrete ideas to modify and expand this hack go here.
Freeform discussion of this hack goes here.
Links to relevant websites go here.
Errata go here.
This is a page for a hack from the book MindPerformanceHacks by Ron Hale-Evans.