Just for the understanding: a + Modifier to the fumble level is a bad thing and makes things worse for the one that rolled it, not better. Treating the 4 and 5 fumble from bad luck the same as a 3 fumble would make things actually not that worse for a player.
There is even a way to get realy lucky and "unfumble" by roling a 3 for fumble and then anythign from 1 to 14 in fumble level- giving you a "fumble"-level of -14 to -1
For Fumble Levels:
Good Luck: -1 Fumble range
Mastery: -1 Fumble range, also eliminates the +2 increase to Fumble range from Complex Weapons
Bad Luck: +2 Fumble range
Compley Weapon: +2 Fumble Range
|good luck & mastery||-15|
|bad luck & mastery ||+30||+15||0||-15|
|bad luck / complex weapon||+45||+30||+15||0||-15|
|bad luck & complex weapon||+75||+60||+45||+30||+15||0||-15|
I made this table based on the "mastery" rule from pages 6 in the core book where mastery reduces the fumble range to 1(0) and 2(-15). The trend used here should be clear so you should have no problems apply other modifieres to the fumble level like those you can get trough magic (arcana exxet: cahos sub-paht) or elan (Eriol: Altering the law of chance).
Uhh, even thinking about all these modifres makes me shiver... imagine a fumble level of 20 (1-20)(3(Standard)+2 (Badluck) +2 (Complex Rule) + 5 (100 Elan Eriol) + 8 (Arcan Alteration of Propabilities)) .... and a open roll range of 39 (61-100)(9 base +10(100 elan Eriol) +20(Arcan Alteration of Propabilities))