I agree with that.

I checked how much more damage my fireball does on my last level up and it is indeed about 10 per cent.

I checked how much damage my Warriors' Fire Rain and Ghost Blade do and thanks to simple mathematics I'm sure that 1 point at Intelligence gives 10% to damage of a spell (lucky me having item giving +1 to Intelligence). Don't know exacly how those bonuses stacks because when I calculated this way damage which should my spells do with my current Intelligence I got lower dmg by 100%+

one thing for sure: if you're a Mage you don't want to decrease your Int although you're doing some weird challenge ;)

If every point of intellect gave your spell a 10% increase then at level 21 your spells would be increased by 210%

You work it out

Not necessarily - that depends on whether the percentage increases are additive or cumulative. If they are additive, the increase due to intelligence 21 would indeed be 210%, but if the increases are cumulative, it would be approximately 640%.

Good point

Edit

Direct quote from the game visable when you hold the mouse over the Intellect icon

"Every 7 points on intellect increase magic damage by 10% and every fifteen points increases the duration of spells effect by one turn"

But didn't they said that this was a translation error? (Page 1 of this thread)

Well, this text is from the older versions of the game, where mage was basically a fighter with the higher magic skill. The game changed a lot since then, but someone forgot to update the text during localization.

The correct one is: Every point of intellect adds 10% to magic damage, every 7 points add additional 10% and every 15 points increase the duration of some spells by one turn.

Intellect not only buff spells damage but other things too.
For example it increase value of precision, pain mirror and others but not all, same for duration i never saw my duration of sheep spell higher than two even with 35 intellect...
This game really needs english manual or wiki :(.

amamake is correct