The problem with this is that you have to know a priori what the range of your numbers is.

suppose $foo = 1e-11 and $bar = 2e-11 your method will say they are not significantly different.

Suppose $foo = 1.1e54 and $bar = 1.1e54 then you would want to say that they are not different, but it is quite possible that a stray bit would cause your method to say that they are different.

iakobski