LightBurn doesn’t know what the acceleration is, so the overscan amount is “manual” (IE, set by you). Technically, percentage of speed is still the wrong way to do it, because the amount of overscan actually required to get to the requested speed isn’t linear - it’s exponential.
That said, since Smoothieware does a good job of managing the power output based on speed, you don’t usually have to have a “complete” overscan move, but you’ll need enough to get you through the initial part of the curve before the tube reaches the firing threshold, and a bit more to get to the stable ionization zone.
With your acceleration as low as it is, I’d try 5%, maybe a touch higher and see how that looks.