This should doable with the Mathhammer, so let's see. First, skip Terminators and any other 2+ save units because that gets more tricky. Secondly, lets discard the possibility of stray Invulnerable or Cover saves to get the initial numbers since they will be the easiest to figure back in. Third, since I'm working with two ST8 AP3 shots from each launcher when using Krak and limiting the question to AS 3+, I can calculate the number of models hit by Frags to cause the same average amount of killing. Fourthly I'll be limiting myself to T3-5 since those are the most common toughness values of most non-vehicle models and I mainly included T5 because there are a peppering of them in almost all armies and because that is about the limit of the effectiveness of Frags.
Now Frags get to be more fun because of the variety of values for toughness and armor saves among the army lists, but they do break down fairly easily. In fact there are only 12 possible combinations available for what I'm doing. NOTE: Each of these are rounded up so that they exceed the 10/9 rate of Krak, see discussion after the list.
- T3/6+ Wound: 2/3, No Save => Need to hit 2 models. Some Dark Eldar & Tyranid.
- T3/5+ W:2/3, Miss Save 2/3 => Need to hit 3 models. Necron Scarabs, Most IG, Standard GEQ.
- T3/4+ W:2/3, Miss Save 1/2 => Need to hit 4 models. IG Stormtroopers
- T3/3+ W:2/3, Miss Save 1/3 => Need to hit 5 models. Sisters of Battle
- T4/6+ Wound: 1/2, No Save => Need to hit 3 models. Ork Boyz
- T4/5+ W:1/2, Miss Save 2/3 => Need to hit 4 models. Genestealers
- T4/4+ W:1/2, Miss Save 1/2 => Need to hit 5 models. SM Scouts/Neophytes
- T4/3+ W:1/2, Miss Save 1/3 => Need to hit 7 models. Space Marines, Necrons, Standard MEQ
- T5/6+ Wound:1/3, No Save => Need to hit 4 models. Ork Warbiker
- T5/5+ W:1/3, Miss Save 2/3 => Need to hit 5 models. Broodlord
- T5/4+ W:1/3, Miss Save 1/2 => Need to hit 7 models. Dethkopta, SM Scout Biker
- T5/3+ W:1/3, Miss Save 1/3 => Need to hit 10 models. SM Biker.
This was giving some round numbers and some of it I might call differently, like #2 says "3", but the real number is "2.5", so I'd probably frag a two man squad of GEQ, but I might not, but all of the model numbers are rounded up to equal or exceed the Krak.. Also, things like the T3/6+ is Tyranid Ripper Swarms and the T3/5+ Necron Scarab Swarms and I know the Necrons and I think the Tyranids both have the Swarms USR so they take double hits from blast and template weapons, which halves the number needed.
This actually is interesting enough that even though I was going to continue this study through how different cover/invulnerable saves changed it, I might just extend the toughness up to 6. There is a trend appearing I didn't expect that might make it possible to create a general rule. I did expect that each plus of toughness or armor save would require one extra model to overcome, but the 5 to 7 to 11! is something that needs to be looked at.
LATER: Okay, did some figuring this morning and it looks like the breakpoint runs at T3/3+, T4/4+, T5/5+. Never Frag T6, but then that tends to be reserved for individual models. Always Frag T2, but then the only ones I'm aware of are Gretchin and I haven't seen Power Armored Gretchin yet, but then you would only need to hit 4 of them. T2 is also off the chart on the trend line because its predicted value would be T2/2+ and that messes with the formula since the Krak become ine