EDIT: Turns out this is useless :( At first I was planning to do it manually, getting attacked and write down the data, now I think I found the exact % of the reduced damage inflicted while wearing protection.  I tried it out with the beefalo damage and it worked. Now to confirm my formula I need to get either pig/swamp monster/bee/hellhound damage inflicted without armor. Any way here is the formula that I used to find out the reduction %:

100-(x/y * 100) =z => x- damage with armor y-damage without armor z-damage reduction

100-(8/34*100)=100-(0.2352*100)=100-23,52= ~76.48% reduction (beefalo takes out 8 hp with armor, the wiki states  

                                                                                                                                       34/hit without armor)

I don't know if the reduction is different on every monster or animal but what I know is that damage dealt by a bee with football helm on is 3 and the damage without football helmet is ~12,75 I found it by using the following formula:

x*100/23,52=y => x- damage with armor y-damage without armor


And to calculate the amound of damage taken with armor is:

23,52/100*y=x => x- damage with armor y-damage without armor ( we will use the bee damage values again)


and we just round the number to 3.

If there is any good mathmatician out there that can confirm this, please do. I would also like to ask you to find out the damage taken without armor of a bee (note you need to get hit twice to see if it's 12 or 12,5/12,75) and post it in the comments.

Thank you for your time