any number you see on your skills without a " % ", means it's a magnitude number, it works with a formula to prevent these debuffs/buffs, to reach 100% or -100%, leaving you at a limit of 75% or -75% working pretty much the same as damage resistance magnitude you get from polarize hull, plating consoles, or aegis reactive shielding for example.

a 42.9 dmg debuff in this case is something arround -29.8% dmg to the opponent

*there are other complicated formulas in the game like the one used by fbp where 1 means 100%