It's pretty easy to match a certain ships speed, either divide the tonnage by engine power to tell you how much power to weight that ship has, then multiply that by the weight of your next ship, if you end up needing an engine bigger than size 50, just Halve the power and use 2.
The other way is to design all your ships with a standard weight ratio and engine, for instance make a 600 ton 70% multiplier engine and put one on your 3000 ton frigate, 2 on a 6000 ton destroyer, 3 on a 9000 ton cruiser, etc. Using this system you can keep the same engine for faster ships, just make them a little lower in tonnage than the standard, ie 2000 ton corvette, 4000 ton light destroyer, 8000 ton light cruiser.
