TBH I haven't tested myself as to whether or not it is front loaded or not. This was just something I was told and why I prefaced it with saying if my understanding was correct.

The thing is that you can't just use it like some knuckle-dragger. Take the unavoidable AOE after Frot rune in Titania. If your party doesn't stack for Frost rune and have the vuln stack then TBN won't pop for this. Testing for these kinds of things and finding when I can and can't use it effectively is the thing I'm finding engaging about it because our resources are much tighter. In SB if it did or didn't pop, I wouldn't really care just b/c resources were so plentiful and it wasn't much of a defensive or offensive gain. Now it is a massive defensive gain while being a offensive neutral. The issue is that people want it to be both an offensive and defensive gain.

As far as the math goes it isn't that difficult. The max damage that you have to take to compare to TBN is just "(max hp)*.25/%" where % is the % of the CD you are comparing. This is a flat amount regardless of timing, number of hits, or when TBN pops. Like taking 100k dmg over 5 hits (so 20k per hit). TBN pops on the 2nd hit (using my 30.7k example) and you take the remaining 3 hits raw. If you compare this to say Sheltron (which is currently blocking for 20%), you'll have all 5 hits mitigated but the overall result will be less mitigation because:
Damage Taken:
TBN-> 100k - 30.7k = 69.3k
Vs
Sheltron-> 100k * 20% = 80k