(Total Engine Output / Class Size) * 1000. So a ship of size 250 with a total engine power of 800 would move at (800/250)*1000 = 3200 km/s
Checking the tutorials if you didn't read them is sometimes useful, or re-reading them just in case.
So if you use either the size of the ship, or the power of a specific engine, you could figure out what size/power output you would need.
For example, let's say we have a size 200 ship. We want 600km/s, thus we have an expression : (X/200)*100 = 6000(km/s)
With simple math, we can deduce that X is 1200. Thus you need 1200 engine power to give a ship size of 200 a 6000km/s max speed.
At least, I'm pretty sure. I'm writing this half-asleep.
(If you want to use tonnage in the expression, each point in ship size is approximately 50 tons, if I'm not mistaken. )
Edit: A Verrryyyy simple method if you're using speeds that are rounded up to the nearest thousand, (6000,7000,etc) just multiply by the size by the number of thousands, and you have your required power output. So for example, 6000km/s = 6. 6x200(size) = 1200 output
