So I've been (casually) leveling a Mercs/Nature Mastermind and because of my build plan, I needed to obtain knockback protection from somewhere other than slotting a Blessing of the Zephyr (https://wiki.cityofheroesrebirth.com/wiki/Blessing_of_the_Zephyr:_Knockback_Protection_(4_Points)) proc in a movement power.
For specifics, the power I needed to slot for knockback protection is Slug (http://web.archive.org/web/20140821081409/http://tomax.cohtitan.com/data/powers/power.php?id=Mastermind_Summon.Mercenaries.Slug), one of the three personal attacks. The single target attack has an 8s recharge and a 1.67s animation time.
Since Slug (http://web.archive.org/web/20140821081409/http://tomax.cohtitan.com/data/powers/power.php?id=Mastermind_Summon.Mercenaries.Slug) can be slotted with Knockback Sets (https://wiki.cityofheroesrebirth.com/wiki/Enhancement_Sets#Knockback), the obvious solution was to reach for the Kinetic Crash (https://wiki.cityofheroesrebirth.com/wiki/Kinetic_Crash) set to achieve the desired knockback protection (-3 mag for 4-slot). However, that then begs the question of whether going to a full 6-slot to reach the 7.5% recharge reduction set bonus is ... optimal.
In this case, the "middle 2" enhancements of the Kinetic Crash (https://wiki.cityofheroesrebirth.com/wiki/Kinetic_Crash) set (Knockback/Recharge, Knockback/Endurance) are technically the "least useful" or otherwise "most easily replaceable" parts of the set. Furthermore, dropping from 6-slot to 4-slot set bonuses drops the 5: +6% Regeneration and 6: +7.5% Recharge global set bonuses ... so what could be "a better investment" for those 2 slots?
Answer ... 2 slots of Force Feedback (Knockback/Damage/Endurance, Proc), which then has an incidental yield of a +7.5% Movement Speed instead (Nice to Have™, but not terribly relevant to the point I'm wanting to make).
Now, in terms of equivalencies, a single Force Feedback proc is "worth" +100% Recharge for 5 seconds. For simplicity of illustration purposes, let's call this "500 points of recharge bonus" to make the comparison easier. By contrast, the 6-slot set bonus of Kinetic Crash is +7.5% Recharge "at all times" that the set bonus is active.
So for the purposes of comparison, how often would Force Feedback need to proc in order to be "equivalent" (in terms of yield) to the recharge bonus of 6-slot Kinetic Crash?
500 / 7.5 = 66.66666667
In other words, 1 proc of Force Feedback every 66-67 seconds will "yield" a (roughly) equivalent amount of return in global recharge as the 6-slot set bonus of Kinetic Crash. That's basically a proc rate of 1 per 1.1 minutes to reach (mathematical, in a vacuum) parity.
So if I were to put a Frankenslot Combo of 4-Kinetic Crash plus 2-Force Feedback into a single target attack power such as Slug (http://web.archive.org/web/20140821081409/http://tomax.cohtitan.com/data/powers/power.php?id=Mastermind_Summon.Mercenaries.Slug) on a Mercs/Nature Mastermind ... what kind of yield would result from that choice?
The numbers below assume use of Level 27+5 (boosted) enhancements and the use of a T4 Radial Alpha slot that adds +11 (ED) and +22 (non-ED) damage bonus.
Kinetic Crash (2: 4.5% Movement Speed, 2.25% S/L Resistance/3.75% Mez Resistance, -3 Knockback Protection)
Knock/Dam (27+5)
Knock/Acc (27+5)
Knock/Dam/End (27+5)
Knock/Dam/Acc (27+5)
Force Feedback (7.5% Movement Speed)
Knock/Dam/End (27+5)
Proc (21)
Accuracy: 21.063+16.85=37.913*1.25 = 47.39125%
Damage: 21.063+(16.85*3)=71.613*1.25=89.51625+11=100.51625 ... 95.0774375+22 = 117.0774375% with T4 Incarnate Alpha, post-ED {87.564625% pre-T4 Incarnate Alpha, post-ED}
Endurance: (16.85*2)=33.7*1.25 = 42.125%
Knock: (37.938*2)+(30.35*3)=166.926*1.25=208.6575 ... 172.298625% post-ED
Recharge: 0
Force Feedback (2.0 PPM)
2.0 * ((8 / ( 1 + 0 / 100 )) + 1.67) / (60 * (0.25 + 0.75 * (1 + 0 * (11 * 0 + 540) / 40,000))) = 32.23%
What that means is that an attack power (Slug (http://web.archive.org/web/20140821081409/http://tomax.cohtitan.com/data/powers/power.php?id=Mastermind_Summon.Mercenaries.Slug)) with a base recharge of 8s and a 1.67s animation time, with ZERO recharge in the enhancement slots would have ... a 32.23% chance per use to proc Force Feedback.
Even with ZERO global recharge (Hasten, set bonuses, Force Feedback procs, etc.), this attack power can be used 6 times within 66.667s ... with each use having a 32.23% chance to proc Force Feedback ... in order to (mathematically) "match" the +7.5% Recharge 6-slot bonus of Kinetic Crash.
8+1.67 = 9.67*6 = 58.02s < 66.667
8+1.67 = 9.67*7 = 67.69s > 66.667
In other words, in sustained combat, odds are "decent" that the Force Feedback procs option (4-KC, 2-FF) will yield "more return on investment" than the alternative of going a full 6-slots in Kinetic Crash ... while still retaining the desired knockback protection set bonus offered by Kinetic Crash.
Net result ... if you don't mind enhancing knockback effects on knockback (capable) attack powers, then Frankenslotting a combination of 4-Kinetic Crash plus 2-Force Feedback (including proc) might be a compelling alternative to other potential options ... particularly in builds that have few (or even just 1!) knockback attack powers.