OK, after having done this for the Nth time, using google to look for the results, I got tired of it all.
So I wrote a spreadsheet to handle the calculations for HE vs IFHE penetrations for arbitrary gun calibers. It's attached to this post.
Simple enter the gun caliber, and whether or not you're in a RN BB, KM CA, or KM BB.
It's updated for the current (Feb 2018) penetration values, and shows possible penetrations vs 16,19, 25, 27, 30, 32, and 50mm of armor, those being the most common thicknesses across the vast majority of ships.
It highlights where IFHE will help.
WOWS IFHE Pen Determination.xlsx