I have two rather similar civilian designs:
Bordeaux class Freighter 39,850 tons 350 Crew 1215 BP TCS 797 TH 1265 EM 0
3174 km/s Armour 1-103 Shields 0-0 Sensors 1/1/0/0 Damage Control Rating 1 PPV 0
MSP 19 Max Repair 86 MSP
Cargo 25000 Cargo Handling Multiplier 5
Magneto-plasma Drive E0.65 comm (11) Power 230 Fuel Use 6.5% Signature 115 Armour 0 Exp 2%
Fuel Capacity 300,000 Litres Range 208.4 billion km (760 days at full power)
Ghoul B class Salvager 39,700 tons 528 Crew 1375 BP TCS 794 TH 1265 EM 0
3186 km/s Armour 1-103 Shields 0-0 Sensors 1/1/0/0 Damage Control Rating 1 PPV 0
MSP 22 Max Repair 200 MSP
Cargo 15000
Salvager: 1 module(s) capable of salvaging 500 tons per day
Magneto-plasma Drive E0.65 comm (11) Power 230 Fuel Use 6.5% Signature 115 Armour 0 Exp 2%
Fuel Capacity 200,000 Litres Range 139.5 billion km (506 days at full power)
The funny thing is that one of my shipyards, tooled for the Ghoul, can build the Bordeaux as well, as per the 20% rule, but the other (tooled for Bordeaux) cannot build the Ghoul. Both shipyards have exactly equal stats. How come?
The BP numbers don't tell the whole story. The 20% rule isn't that the BP costs are within 20%, it's that the cost of refitting from one to the other is less than 20% of the total build cost.
Let's look at the (obvious) changes in each design.
From Ghoul to Bordeaux, you must:
Add 2 Cargo holds (10 BP each)
Add 1 Cargo Handling System (10 BP)
Add 2 Fuel Storage (10 BP each)
An addition of 50 BP
Going from Bordeaux to Ghoul:
Add Salvager 500 (200 BP)
Add 1 Crew Quarters (15 BP)
An addition of 215 BP.
Now, since you need to remove the salvager and crew to make the Bordeaux, and the cargo, etc for the ghoul, the total difference in BP is 50+215, or 265 BP.
265 / 1215 = 21.8%
265 / 1375 = 19.2%