I really don't get the problem?
Set up two identical atomic clocks sit them side by side and check they hold accuracy for a year. Now send one of the clocks up to the ISS and when it comes back in a couple of months check it .. THE CLOCKS ARE OUT IT'S BEEN DONE SEVERAL TIMES.
It's a repeat of the Hafele–Keating experiment done back in the 70's where they flew an atomic clock around in a plane.
(
https://en.wikipedia.org/wiki/Hafele-Keating_experiment)
You don't need to go to 0.8c to see the effect.
That experiment has been done several times in different version on the ISS ... search it.
There are actually two dilations on the ISS which is why the experiments have been done so many times it extends the original Hafele–Keating experiment.
First time goes faster on the ISS because they are in lower gravity of earth (89%). Second time slows down for them because they are moving so fast (27,600 km/h).
The two effects are opposite but for them the slower time dilation based on speed dominates but scientifically they wanted to check the two effects are truly additive.
As for true measure of distance Bill S find us a patch of space that isn't moving and we will do the test for you
Incase you don't get the problem our patch of space is hurtling around the milky way galactic centre at 828,000 kilometers per hour. When you are in transit to you other celestial body I have no idea how you are going to work out what you actual motion is, at the end of the day you are heading to a target that is all you have told me. I have no way to know the motions of the space between those two bodies without more detail. If the two bodies are both in the milky way your motion can never be zero as you have the galactic spiraling to worry about. All I can say is earth is probably not a good area because we have an inbuilt velocity just to stay in position within the galaxy.
Now if you could actually stop motion totally you can move at 828,000 kilometers per hour in an arc in our own galaxy
