I play as a mage (my first serious game) and I am absolutely sure - every point of INT increases the damage with 10%; every 7 points give 10% additional bonus; so with 9 points if INT you get double damage. Minus the enemy resistance of course. I checked it at the very begining. Non-modified (with 0 INT ) fireball makes 155 damage in the central hex; with 1 INT it makes 170 damage which is exactly 10% increase (rounded down).

INT increases the damage and from spells like "mirror pain". The basic value of the damage is percentage from the damage dealt by the enemy in the previous turn. In this case what the INT makes is to increase this with 10% FROM THE BASIC VALUE for each point of INT. So if the basic value (for example) is 60% then each point of INT increases it by 6% Don't know if any enemy resistance counts when you cast "mirror pain". The magic resistance if any should be counted but don't really know.

From the Manual
 

Formula for damage for spells says :

intellect's modifier : (1+0.1*int)*(1+0.1*(Int/7 "<- only integer part"))

ie. Int 15 = 2.5*1.2 = 3 (extra turn duration on some spells)
ie. Int 30 = 4*1.4 = 5.6 (2 extra turns duration on some spells)

note: some spells are capped on maximum duration, others do not get a bonus at all. ie. dragon arrow give 1 additional arrow every 15 point, up to 5 arrows. Hypnose duration is 2 rounds no matter what, etc

Total damage will be : base dmg of spell * intellect's modifier * item's modifier<- (sum of items/ability bonuses) ; remember to apply resistance from target

Note: Sacrifice doesn't add the extra 10% per 7 ints ; pain mirror just adds int/2 to %dmg returned.

So, answering your question, no, it isn't just 7 and 15, every point matters, but not all points matter the same
IMO: throw that garbage away as soon as u get something better, or if your strategy is heavily based on spell's dmg.

I use 1.65 version and it seems the INT adds more to the pain mirror spell. At first lvl spell with 34 INT I have 129% return. The basic value of the spell was 75% (I think). So those 34 points of INT add 50% of damage. Obvous not the 6% as I thought but much more then the 17% from INT/2.

129% is a pretty weird number :) yes, the base is 75%, so you're supposed to have 75%base *x +17% returned dmg , where x is your bonus from items/ability. So, in this case, 129 = 75x + int/2 -> x = 1.5 , meaning 50% from item's bonus... i can think of destroyer level 3(50%), just my guess. am i right? I have too the 1.65 installed (manual referred to 1.64, but i don't think they had any reason to change this), but i can't verify, cause too much snow to go back home :( i miss my pc so much!! hope to be helpful, let me know

Yes, I forgot about the destroyer lvl 3:-)