A spaceship in orbit around Earth and deaprts out into space. On board I can measure acceeration and calculate change in velocity. I turn off the engine and the ship is traveling at a constant speed.

I have a number for velocity but it is relative to the starting point. I get the same change in velocity regradless of the velocity of starting point.

From relativity there is no absolute frame, so it is impossible to know an absolute velocity of a starting point. Given that, on the ship how do I know what percent of C the ship is traveling at?