Increased spell damage

I don't get it. I have lighting spell damage 7721 DPS. I put Scold's bridle with 99% spell damage increase. And my DPS it went only to 7775. So increase is 54dps? Is it ok? With 99% spell increase?

1. Tool tips can be wrong.

2. You are looking at DPS, that is damage per second. You increased damage while keeping the cast time the same. You won't see a 99% increase in DPS.

You have to see the difference between "increased" and "more" ("reduced") here.

The item you are using has a increased modifier wich means it is additive with every other increased damage modifier you have and is then multiplied with your damage. This means it becomes less and less effective with each other spell damage or elemental damage node you took.
The negative modifier on the item on the other hand is multiplicative wich means it is a flat 15% less castspeed --> flat 15% less dps.

To translate this into an example:
Lets say you have between passives and your wand+shield 400% increased spell damage and with this item you push it to 500% this will be an increase of 25% (equal to a 25% more mod). Now we reduce the dps by 15% (for the castspeed) wich means 500%*0.85= 425 so you can now calculate the actual increase in dps 425*100/400 = 106,25 --> you only get a dps increase of 6,25%

Scold's make u hit very hard but less frequent, resulting in same dps as before. Since cast speed and crit multi(for crit builds obv) r the best stats to get high tool tip dps for spells and u decreasing one of them, u r not gaining as much, as it looks. Especially with already mentioned decreasing effectiveness of %increased when u stack it.

Thank you all for comments.