I just modded the drive mass in my game like that:
<ShipComponent>
<InternalName>HyperDrive</InternalName>
<DisplayName>HyperDrive_Name</DisplayName>
<Description>HyperDrive_Dec</Description>
<ArtDefine>Hyperdrive_01</ArtDefine>
<Category>Engines</Category>
<Type>InterstellarDrive</Type>
<PlacementType>Drive</PlacementType>
<Stats>
<EffectType>Value</EffectType>
<Target>
<TargetType>Ship</TargetType>
</Target>
<BonusType>Flat</BonusType>
<Value>0.25</Value>
</Stats>
<Stats>
<EffectType>MovesCapManufacturingCost</EffectType>
<Scope>Queue</Scope>
<Target>
<TargetType>Ship</TargetType>
</Target>
<BonusType>Flat</BonusType>
<Value>6</Value>
</Stats>
<Stats>
<EffectType>MovesCapMass</EffectType>
<Target>
<TargetType>Ship</TargetType>
</Target>
<BonusType>Flat</BonusType>
<ValueType>Special</ValueType>
<SpecialValue>
<Special>HullMassScaleMod</Special>
<ValueParam>-5</ValueParam><!-- from 8 -->
<ValueParam>0.4</ValueParam><!-- from 0.2 -->
</SpecialValue>
</Stats>
<Stats>
<EffectType>MovesCap</EffectType>
<Target>
<TargetType>Ship</TargetType>
</Target>
<BonusType>Flat</BonusType>
<Value>1</Value>
</Stats>
<Stats>
<EffectType>Maintenance</EffectType>
<Target>
<TargetType>Ship</TargetType>
</Target>
<BonusType>OneTime</BonusType>
<Value>0</Value>
</Stats>
<Prerequ>
<Techs>
<Option>EngineeringTech</Option>
<Option>TorianEngineeringTech</Option>
</Techs>
</Prerequ>
</ShipComponent>
That worked not really like expected (drive mass = -5 + 0.4 * 36 = 9.4; I get 5.8, implying that the formula used is -5 + 0.3 * 36; 36 = tiny hull size of the Drengin at start of game):

When I look at the modded values for Hyperdive Plus:
<ShipComponent>
<InternalName>HyperdrivePlus</InternalName>
<DisplayName>HyperdrivePlus_Name</DisplayName>
<Description>HyperdrivePlus_Dec</Description>
<ArtDefine>HyperdrivePlus_01</ArtDefine>
<Category>Engines</Category>
<Type>InterstellarDrive</Type>
<PlacementType>Drive</PlacementType>
<Stats>
<EffectType>Value</EffectType>
<Target>
<TargetType>Ship</TargetType>
</Target>
<BonusType>Flat</BonusType>
<Value>0.5</Value>
</Stats>
<Stats>
<EffectType>MovesCapManufacturingCost</EffectType>
<Scope>Queue</Scope>
<Target>
<TargetType>Ship</TargetType>
</Target>
<BonusType>Flat</BonusType>
<Value>10</Value>
</Stats>
<Stats>
<EffectType>MovesCapMass</EffectType>
<Target>
<TargetType>Ship</TargetType>
</Target>
<BonusType>Flat</BonusType>
<ValueType>Special</ValueType>
<SpecialValue>
<Special>HullMassScaleMod</Special>
<ValueParam>-5</ValueParam><!-- from 8 -->
<ValueParam>0.3</ValueParam><!-- from 0.1 -->
</SpecialValue>
</Stats>
<Stats>
<EffectType>MovesCap</EffectType>
<Target>
<TargetType>Ship</TargetType>
</Target>
<BonusType>Flat</BonusType>
<Value>1</Value>
</Stats>
<Stats>
<EffectType>Maintenance</EffectType>
<Target>
<TargetType>Ship</TargetType>
</Target>
<BonusType>OneTime</BonusType>
<Value>0</Value>
</Stats>
<Prerequ>
<Techs>
<Option>InterstellarTravel</Option>
<Option>TorianInterstellarTravel</Option>
</Techs>
</Prerequ>
</ShipComponent>
I expect a drive mass of -5 + 0.3 * 36 = 5.8, but I get the double value:

Can somebody tell me what is wrong?
Edit: I will never understand how inserting images works on this forum 
I don't see the images in my post, but they appear when you click on it (at least for me).
Edit 2: Thanks to Old Spider the images are now shown 