What's the most reliable way to work out your caloric maintenance?

Take their average

http://www.exrx.net/Calculators/CalRequire.html

>>42502345
nsuns tdee spreadsheet, use it for 2 weeks or so and you'll get an accurate TDEE

calculators don't take into account variation due to underfeeding or overfeeding so your best bet is to just base your tdee on changes in your weight with a known amount of calories

You'll never have an accurate number. There's just too many factors that can't be numerically accounted for when programming calculators.

They are useful in providing an approximate number but to nail it down to be more accurate, you'll need to pick a number and trial and error till you get it.