I don't have any first hand experience of calibrating a valve tester (yet) but there seems to be a well-known problem with the magnets in the meters losing their field strength. I imagine that this means they read low, which sounds like your problem. There's a discussion here
http://www.radiomuseum.org/forum/wha...be_tester.html and there may be much more info available on this. Maybe someone who knows more than I do can advise ?
Cheers,
GJ