i'll redo the maths after testing how BC works on the practice tool. BC reduce both base and bonus armor by 5% per stack, so a 5% total armor reduction per stack, not with bonus armor being prioritised:
LW alone makes the total armor 68.5 (30 / 38.5)
1st hit: only BC: 95 (28.5 / 66.5). With LW: Armor Penetraded = 65.075(28.5 / 36.575)
2nd hit: Armor shreaded = 90 (27/63 ). With LW: 63.13 (27/33.13)
3rd hit: Armor shreaded = 85 (25.5 / 59.5). With LW: 58.225 (25.5/32.725)
....
Its best to take BC fully stacked + LW as having the enemy with 70% base armor and (0.7*0.55) = 38.5% bonus armor, and then theres the flat armor from lethality to also keep in mind. it adds up fast.
Having both LW and BC is overkill unless the enemy team has 2+ tanks and is stacking armor like crazy, but its still quite efficient against those and shouldnt be disregarded. If there is only one tank, its best to ignore it and just focus on roaming and on their backline of course.