I'v played ALOT games with similiar build on Jarvan, with one diffirence thou, getting 1.TriForce 2.Mercs 3.Warmogs. 4.Atmas. 5.Veil and as said on murtaghking's comment, LW is only more effective than BC when enemy has great amounts of armor, such as tanks, witch u shouldn't be targeting in the first place.

Therefore you should consider, atlest as an optional build, where you have BC instead of LW, and build little more balanced through the game imo :D

nice build but i would recommend you to take last whisperer only if the enemies have more than 120-150 armor otherwise black cleaver is better

about your Q: first reduction than penetration
-> example : 100 armor (26% reduction from Q) -> 84 armor now your armor penetration works on the 84 armor

+1 good guide