The leech % is an equation not a fixed number. It's really an odd ball method they developed. The leech % on the weapon determines the maximum value for a random number. Then upon a successful hit a random number is generated inclusive between 0 and the leech%. Then you get 40% of the damage back as mana. You do not include the spells that go off or velocity or any other affect only the base weapon damage result.
No matter the order the SSI is applied the leech will adjust because the number displayed is calculated not fixed.
http://www.uoguide.com/Hit_Mana_Leech
Just to talk about leeches further...
You want to add the maximum amount of HML that you can afford in imbuing weight to move the median of the leech amounts upon successful hits higher. This one needs to be higher because each successful hit the random number is generated and sometimes the leech will be 0 and sometimes it will be 10 and sometimes it will be 34 or any number in between (assuming 34% max HML). Then you leech 0.4 times that random number as a percentage and times the damage on the target.
This is contrasted to Hit Stamina Leech where you want to choose the leech% as the number of successful leeches (procs) in a succession of successful hits. The hit stamina leech will give stamina to the full damage amount (not the hit spell or velocity or other), but only is successful a percentage of the time. So let's say you measure 10 swings and hit the target 70% of the time. If your HSL is 34% then 34% of 70% will have leech events so about 2 times out of 10. So you want to choose the HSL based on typically how close your stamina buffer is to the minimum desired stamina. In reality you just need to keep the median stamina above that threshold that affects your swing speed. Say I am a sampire and I can go down to 90 stamina and still swing at 1.25s. I just need a couple successes to actually get my stamina back full refreshed to 182 stamina because my damage is huge like sometimes 300 pts.
-Lorax