Average mass per block should be about 0.0002 mass
dV is your change in velocity per second, and to find your potential dV, I am using
dV = Thrust/Mass
To calculate thrust, let x be your total number of thrusters,
Thrust=LOGe(# of Thrusters)+1
With this set up, each thruster gives reduced thrust per block, and because of their mass, you can actually reduce your dV by adding to many thrusters.
Here are some example ships from my spread sheet:
A \"small\" ship with 500 blocks, and an additional 110 thrusters, for a total of 610 blocks. This ship has a mass of 0.122. It produces 5.7 thrust, for a dV of 46.72 kph. It will accelerate to 100kph in a blazing 2.14 seconds!
A large ship with 100,000 blocks, and an additional 11060 thrusters, for a total of 111,060 blocks, and a mass of 22.2. It produces 10.3 thrust for a dV of 0.46 kph.It will take about 56 seconds to accelerate up to 25kph, 1.7 min to get up to 50kph, and just over 3.5 min to get up to 100kph.
A huge ship that has 1,000,000 blocks, and an additional 90,000 thrusters, having a total mass of 218, will produce 12.4 thrust, and have a dV of less then 0.06 kph. This monster will take 3 min to reach a mere 25kph, and to reach 50 kph will take 7.3 min. Just before 15 min, you will finally make it up to 100kph. With something this big, you would have to plan to stop well in advance.
These figures are based off a very quick formula, and would have to be tweeked, small ships get to much thrust, large too little. But with these figures, planets should have a gravity that create a dV of between 0.2 kph to 0.5 kph. This would make it so only smaller ships can be used to enter a planets gravity, as large ships would not be able to lift off.
Ship cores could have a mass of 0.5 reducing the acceleration of the \"small\" ship I described so that it takes a full 11 seconds to reach 100 kph, while not really affecting the larger ships in any way.