Tier 3 (1302)
+1/2/3/4/5/6 upgrade are all 100%
+7 upgrade is 60%
+8 upgrade is 45%
+9/10/11 upgrade are all 30%
+12/13/14 upgrades are all 15%
+15 upgrade is10%

Tier 3 (1340)
+1/2/3/4/5/6 upgrade are all 100%
+7 upgrade is 60%
+8 upgrade is 45%
+9/10/11 upgrade are all 30%
+12/13/14 upgrades are all 15%
+15/16/17 upgrades are all 10%
+18/19 upgrades are all 5%
+20/21 upgrades are all 3%
+22/23 upgrades are all 1%
+24/25 upgrades are all 0.5%

The chart is what you are UPGRADING TO, not what your upgrading FROM. In this case your upgrading TO +12 so the chart line is the +12 line. Thats pretty much how ALL the charts are as its giving information on what it costs to upgrade to that +.

And as stated, you can see the % chances in game along with the modifiers.

Did they change the base chance at some point in game history then?

Because all the google charts I find say 30% base for +9/10/11 on 1302 gear.

I believe ppl saying it is 15%, because clearly the math works that way from what I see in game today (45% adjusted chance on +11 to +12 attempt = 15 base + 20 growth buff + 10 SH buff)

No and we just stated why. you are trying to do a +12 hone. LOOK at the +12 line. The charts are BASED on what you are HONING TO, not from. You are honing FROM +11 to +12 meaning you look at the +12 line for the cost and chance. It has been 15% base chance for +12 since launch.