Percent, which is per cent, where cent is like century, or 100, so per 100, or /100.
So 1% is 1/100, or 0.01. 1.23% is 0.0123.
I think that is all you need to know. 100% = 1.
I would store a percentage as a number. Double is usually too much precision for what I do, so usually a single would be plenty.
Hope that helps,
formerly known as lagbolt | Windows 10 | Access 2010 | Visual Studio 2013 | "Institutions have a vested interest in perpetuating the problems to which they are the solution." - Clay Shirky