The general formula for the distance between two parallel lines \( ax + by + c_1 = 0 \) and \( ax + by + c_2 = 0 \) is:
\[
\text{Distance} = \frac{|c_2 - c_1|}{\sqrt{a^2 + b^2}}.
\]
For the lines \( 3x + 4y + 1 = 0 \) and \( 6x + 8y - 1 = 0 \), the coefficients of \( x \) and \( y \) are proportional. The lines are parallel, so we can use the distance formula for parallel lines.
Thus, the distance is:
\[
\frac{|(-1) - 1|}{\sqrt{3^2 + 4^2}} = \frac{2}{\sqrt{9 + 16}} = \frac{2}{5} = 0.(3)
\]