At 120 Mysticism + 120 Focus or Imbuing (whichever is higher) you should receive Spell Channeling -1 (if the weapon doesn't already have spell channeling) and 60% hit spell chance. It's currently giving Spell Channeling -1 (if the weapon doesn't already have spell channeling) and 50% hit spell chance. Tested, and compared both tonight. Will go back and get screenshots when I get a moment if necessary.
Also see:
"https://uo.com/wiki/ultima-online-wiki/skills/mysticism/
Adds up to 60 Hit Spell of your choice (Fireball, Harm, Magic Arrow, Lightning, Dispel) to a weapon. If Focus or Imbuing are over 80 it also adds Spell Channeling -1 Faster Casting. Duration scales based on the level of the spell to be enchanted, base duration 150 seconds.
The spell is cancelled if the weapon is disarmed "
Also see:
"https://uo.com/wiki/ultima-online-wiki/skills/mysticism/
Adds up to 60 Hit Spell of your choice (Fireball, Harm, Magic Arrow, Lightning, Dispel) to a weapon. If Focus or Imbuing are over 80 it also adds Spell Channeling -1 Faster Casting. Duration scales based on the level of the spell to be enchanted, base duration 150 seconds.
The spell is cancelled if the weapon is disarmed "