Casting Ebb abilities

This takes two phases, one to formulate the ability and one for the ability to manifest. The basic attempt is done by a success roll of 2d10 + Formula - FLUX cost. This keeps it nice and simple for the easy abilities. For more complicated abilities they can expend more FLUX giving a +1 to the roll for each additional FLUX used (ie: not including the amount required for the ability in the first place). The reason that the amount of FLUX works as a modifier is that JNFWoP sees FLUX as a fluid and the more you try to compact into an area, the more will try sneaking out between any available gaps. Additional FLUX can be used to keep replenishing the FLUX supply until the ability is manifested, giving the ability more chance to succeed.

Example : Char wants to use Force Ebb at Rank 4 (ie: FLUX cost 4) His Formula is rank 3 and he has 10 flux available
Standard roll is therefore 2d10 +3(Formula) -4(FLUX cost) ie: 2d10 -1
If he wanted to use all of his FLUX on this one ability he could get 2d10 +5

More complicated abilities

If the Ebon has more time they can concentrate and channel higher levels of FLUX. This requires a greater level of concentration by the ebb user, similar to aiming using marksmanship. For each Phase spent concentrating the ebb user gains a +1 to the roll up to their CONC level. This reflects the fact that the Ebb User is mentally constructing the 'vessel' that will hold the required amount of FLUX for the ability. Again they can still use additional FLUX to add bonuses as well.

Example : Char wants to use Reality Fold rank 10. He still has 10 FLUX available and has a CONC of 7.
His standard roll would be 2d10 +3(Formula) -10(FLUX) ie: 2d10-7, which he'd be hard pushed to do.
Instead he spends 7 rounds concentrating (max amount due to CONC) and gets to roll 2d10 +3-10 +7 ie: 2d10 +0, which is more likely to work. If he has any DeathSuit FLUX he wants to use he could further increase the roll.

Ritual use

For even higher levels of FLUX, the ebb user must undertake a ritual. This helps to fully focus the Ebb in the area, altering it so that it is not trying to escape from the user. Continuing the Fluid analogy, this is making the Ebb turn into a solid item. This again uses CONC in a similar way to marksmanship. Each minute spent concentrating gains an amount equal to Formula, up to a number of minutes equal to their concentration. Again the ebb user can use additional FLUX to add bonuses as well.

Example : Char is now a Necanthrope called Ash. His CONC is still 7, but his Formula has increased to 9. He wants to use Possession on a hapless person that is tied up in his lair. The cost for this is 50 FLUX
His standard roll would be 2d10 +9 -50 which he probably wont be able to do. If he concentrates he can push this up to 2d10 +9 -50 + 7, which still isn't going to work.
So as the victim is subdued and next to him, he starts a ritual. 7 minutes later and his success roll becomes: 2d10 +9 -50 +(7*9) ie: 2d10 +22 which is more than enough to succeed...

Rolling for success

In general if the roll is above 11 it works, if under it fails. It is up to the GM whether rolls over twenty and under 0 do anything spectacular. They are also free to add/subtract numbers to the roll depending on the Ebons closeness to certain buildings/places (ie: +3 if by Guidance, -3 if on Artery etc). For a more limiting penalty have areas that modify the Ebons Formula, rather than just their roll.

Extra notes:

It does have a problem with high cost powers, but it also stops people doing things like total regeneration or possession in one phase of a combat round. It makes the story more interesting, because high-level powers have to be planned in advance, and not just used spur on the moment.

The flux cost was used instead of the rank of the ability, due to the principle that if flux is a 'force' then the more you try and use it, the harder it becomes to control it. I guess it would be up to the GM. If you want to reflect that the Ebb user is better at abilities he has a higher rank in then, you could allow +1 for every five ranks in the ability. That way Formula would still be the more useful. An alternative could be no bonus for instant spells, +1 per 5 ranks for concentrated spells, and +rank for ritual spells (because these won't end up with enough bonuses as it is...)

One thing I forgot to mention was that each 'upgrade' of the DeathSuit increased your roll by +1 as well ie: Medium +1, Heavy +2, etc. It was meant to reflect the fact that your DeathSuit makes it easier to calculate Ebb-Abilities than a deck of Glyph Cards. There is possibly a glyph card that can help you use your abilities if it is channeled through the card, adding to the success rate, but I haven't worked out all the JNFWoP rules to glyph cards/ferals.

Something else that we played around with was limiting the maximum level of your abilities by Formula, depending on which Ranks you wanted to concentrate on. Two abilities were chosen as your primary ability and these could be learnt up to a level of Formula +10. Four abilities were chosen as your secondary ability and could be chosen up to Formula +6 and the remainder counted as tertiary abilities and could be taken up to Formula +2.