Super high speed combat

[Condottiere]

Consider gravity like friction.

Or a slingshot.

 

[AnotherDilbert]

It gets quite complicated and hardly usable at the table ...

Quite, which is why there are simplified movement systems from LBB5 to MgT.

A one dimensional system is not all that complicated.

An easy way to get an approximation of the number of combat rounds would be easy.

The regular travel time formulas give a good approximation for Ship A. The time to, and vectors at, intercept depends entirely on the intercept strategy Ship B chooses, so can't be easily estimated.