To get optimal damage from abyssal damage mods you want one(1) module with high Rate of Fire (RoF) and the rest to be high Damage Multipler (Dmg). You want the focused stat as high as possible while having minimal losses to the other stat. This is a result of how EVE does decaying returns.

Notice the percentages on these mag stabs: even with maxed gunnery skills they do not meet the requirements for a bastion badge. The example magstabs are shown below.

Note the lower percentages on these mag stabs and how they create higher actual DPS due to focusing on one stat each rather than splitting it.

As you can see the second set of focused modules, even though they are a 27.5%, 27.8% and 28.9%, will meet the 3900 dps requirement for the Kronos. Meanwhile the ones chosen based on higher percent alone at 28.9%, 29% and 29.1% only do 3893 DPS and are hence not enough. You will also find that because of this it is often cheaper to buy a focused set than a percent based set.

A note on Vindicators: Do to the Vindi not having the burst aerator (increases RoF) or a bastion module it is much less penalized for stacking RoF. As a result optimal for a Vindi is 2 high RoF and 2 high Dmg modules.