Its easy.
And its done via Weapon Fusion not Tempering.
You take your base Weapon, you choose another weapon that you want to use to increase your Base weapons stats with.
You search for your base weapons original attack
You Up the attack of your Base weapon using the second weapon and you search for the new value, you keep doing this until you find the correct values that match your Weapons attack power.
The max amount of attack you can do on a weapon is 255
Though going that high may be a tad pointless as all characters have a stat cap that cant go any higher when its reached.
Yeah, mine won't go over 1800 attack. Even using the CE Trainer posted in the other thread, the max value you can set is something like 16364, but the character info still only shows 1800 max.