https://science.nasa.gov/mission/voyager/where-are-they-now/ says the furthest is 24.4 million km from earth. Pulling out the trusty qalculator:

    $ qalc '24.4e6 km / 1 week / c to %'
    (((24.4 * (10^6)) kilometers) / (1 week)) / SpeedOfLight = approx. 0.0134572816%

So this is using the "millions of kilometers per week" as a speed unit (like km/h but bigger), then dividing by the speed of light (c) to get the fraction of c this is, and finally asks it to format it as a percentage

Edit: found a potentially easier (more intuitive to understand) query, giving the same result, as well as how to ask it nearly for the how manyth part of c this is. I'm still learning all the tricks of this tool :)

    $ qalc '24.4e6 km / 1 week to c%'
    [...] approx. 0.0134572816(%c)
    $ qalc '24.4e6 km / 1 week to 1/c'
    [...] approx. 7430.92125 c^-1