You need to consider the mass, plus when they may have begun charging.
If it's a fairly larger ship than you, then it's going to get away pretty easily. For example, if you're in a small Eagle and the NPC/Player is in a Python, you're not going to do much to their FSD disruption. Remember, it doesn't really prevent jumps, just prolongs it, the lower the mass of the one disrupting it, the much, much quicker it charges. You do not prevent it, the charge is just prolonged with some formula (I don't know what it is, FDev does but it obviously involves the mass of ships involved) which will determine that.
Mass locked is not the same as FSD disruption. Mass locked prevents jumps in its entirety, no ship in the game has the mass to lock another ship like a station does, mass of current ships can only disrupt the FSD charge rate. These are two different mechanics that are being confused with each other and both not understood.