Exact Weapon Upgrade chance (%) to +6/+7/+8 with WS?

[Leamee]

Hey guys,

My WS is now 99/70 and upgrades 10% better than NPCs.
I don't know what that means though - I managed to make 3 +6 Burning bows in a row but struggled to get to +7, and got my first +7 staff of recovery in a single attempt. It took me 3 attempts to get staff of piercing to +7.
I guess this means I'm pretty lucky, but I don't know the exact % for each upgrade.
I'm not attempting +8 as it's a stupid goal given my funds and the amount of oris I have, but I started selling overupped weapons and I hope to make some money out of it as I spend quite a lot on oridecons and weapons.

http://irowiki.org/wiki/Refinement_System

Basically there's a 1.92% chance of upgrading a given weapon to +8 with the regular NPC (= not much).
Does that means that there's a 2.1% chance that a WS could forge the weapon to +8?
It seems like it given that you need 52 weapons to reach +8 with NPC and 52/10 = 5 or so, 52-5 = 47 and 100/47= 2.1%
Not that 0.2% helps a lot but I was wondering if my calculations were correct.