Sources
Where a calculator depends on something outside pure geometry, that dependency is recorded rather than absorbed silently.
What counts as a source here
Standards and official documentation, manufacturer product documentation, recognised trade associations and institutional references. A competitor page ranking well in a search engine is not treated as authority for a number, however convenient that would be.
Where a source is not available
If an external value cannot be supported well enough, it is made a user input, labelled as an assumption, or left out. It is never invented to make a calculator look more complete than it is. No external source is cited on any calculator today, so the Sources block on each calculator lists the planning defaults it starts with instead - the waste or overage allowance, and the number of coats - named as planning defaults rather than dressed up as sources.
Corrections
If a figure here looks wrong, it may well be. Calculation defects are treated as the most serious kind of problem on this site: they are reproduced, root-caused, fixed with a regression test, and checked against every other calculator that shares the same code.