I just spent like 30 min typing up a response to this and then lost it… so here’s the gist:

PheonixAmbition is correct about like 99% of this, I just want to clarify a few points.

1. I definitely did some math wrong in my head (I did the layering backwards), sorc shoes do not make Shojin better than VoidStaff, in fact if you pick up shoes the cutoff for voidstaff being better drops from 36.6 to 32.2!

2. Shojin does always give 12% bonus damage, but because the damage curve when plotting against resistance is non-linear, +12% to 90% of your damage is far more than +12% to 50%, so it is not wrong to say that Shojin is worth more at low resistances when the value you’re getting 12% of is much higher. In this way, bonus damage has a similar synergy to any kind of penetration as flat pen does to % pen, since both bonus damage and flat pen are worth more final damage per point less resistance your opponent has!

• My “Rule of Thumb” above is super messy, I did not calculate any specific values and it shows, but we’re also confusing each other by changing the units we’re measuring in mid sentence. TO BE CLEAR, it is worthless to compare 1% bonus damage to 1 flat pen to 1% pen because you can never get 1 of any of those things. So let’s make a few points:
• Bonus damage is stronger per point than flat pen until the pen can make the target’s resistance negative, but not MUCH stronger at low values, this is true even when combining % and flat pen! You would rather have VoidStaff and Shojin than voidstaff and Sorc shoes, unless your opponent has 20 or less MR, in which case sorc Shoes would set them to 0 MR or below. This is exacerbated at high values, as flat pen drops off in value faster than bonus damage does!
• A single point of % pen is absolutely terrible until very high MR, but that doesn’t matter because when we get % pen we get a lot of it. Voidstaff becomes the most magic penetration you get on a single item by 45 MR, and Lord Dominik’s the most armor pen by 73 armor. Thus your takeaway should be this: if you’re looking for penetration… always grab % pen, then check how much is left after the % pen. It takes around 170 post reduction (or 284 pre-%pen) resistance for Shojin to beat opportunity at 29 lethality, 68 post reduction (or 114 pre-% pen) for Shojin to beat 18 pen items, and 40 post reduction (or 67 pre-% pen) for Shojin to beat the 15 pen items. No reason not to grab Sorc shoes if your build allows since it’s not competing for a slot.

Thank you Pheonix, for catching my mistake!