Is it subtraction like (AP-DR)*blabla or division like (AP*blabla)/(DR*blabla)? 

According to the document below I think it's the former, is that right?

https://docs.google.com/document/d/1rUH18hgXH0EZLDar6GgexjbCWuRaGsmb5dU8xQ8yrtU

So does that mean there is a threshold, only the exceeded AP are "valid AP" and 10% gear AP in solo black shrine actually weighs more than it appears? Like, for example aura gives 318 AP, then they can set bosses DR at 200+ to easily make gear matter way more than it looks like, am I correct?

Cuz I did bs with both my proper gear and tuvala gear, the difference is just huge and I feel there's clearly a "valid AP" threshold which is only slightly lower than the aura AP. Just curious.

Doing black shrine bosses with two different geared characters and yes the difference seems huge, but only regarding DP.

My main gear had a huge AP increase in the last weeks without resulting in much better solo black shrine times, so in my opinion the 10% AP do not have a lot of impact, but other stats like crit damage % and back attack damage % do (garmoth heart crystal slot, cups, relicts, crystals etc)

The above poster is correct. 10% of the current gear does not matter. 

All the dif. gear updates Sov. Kara.. I have had 25 points added with AP. Also having worse clear times with better gear lol.

DR is a straight reduction to ap. But you can never do less than 5% of your total damage/ap.