Back to the original question....
For a rule of thumb, I prefer 1 as its easy to remember and slightly conservative.
I remember seeing 1.25 in one of the CPL text books, but this may be for a distance in stat. miles.
If the VOR is positioned significantly higher than the surrounding land then you can add a bit for this, as bookworm says.
Given the variability with local geography, I'd go for the conservative factor.
This is only for planning anyway. If you have a flag and ident, and are less than DOC distance, then it's OK to use it.