General

[quote=TheDevilish0ne]If you leave it at level 5 as opposed to the max of 15, you will be missing out 20% critical rate as well as 10% minimum critical damage.

Based on my calculations,

[b]Critical Magic Level 5[/b]

Base Critical Rate : 5%
Spell Mastery : +10% Critical Rate
Critical Magic level 5 : +10% Critical Rate, +5% minimum damage

Critical Rate : 25%
Critical Damage : 125%~150%
Average Damage Boost from Criticals : (0.25*(0.25+0.5)/2) = 0.09375 = 9.375%

[b]Critical Magic Level 15[/b]

Base Critical Rate : 5%
Spell Mastery : +10% Critical Rate
Critical Magic Level 15 : + 30% Critical Rate, +15% minimum damage

Critical Rate : 45%
Critical Damage : 135%~150%
Average Damage Boost from Criticals : (0.45*(0.35+0.5)/2) = 0.19125 = 19.125%

So the difference is 19.125% - 9.375% which is 9.75% damage.

In summary, you are losing [b]9.75% damage[/b] if you have 10 less points in Critical Magic due to the lack of the Mastery Book.[/quote]

First off, I'm surprised to see someone else actually write calculations up. Props.
There's a few changes I'd like to make though.
1) The % you have are correct, but I like to make it a multiplier. I.e. 9.375% -> 1.09375x and 19.125% to 1.19125x. I'll explain this in point 2.
2) You can't exactly subtract damage increases like that. I'm not doing 10% more damage if say, my damage goes from 1.1x to 1.2x. I'm doing 1.2[b]/[/b]1.1 = [b]9.1%[/b] more. This is verifiable, especially with %stat. Ex: From 60% to 90% stat, I'm not getting a 30% increase in my damage range or whatever. I'm getting a 1.9/1.6 (as without %stat I have "1x" my base INT) = [b]1.1875x[/b] or [b]18.75%[/b].
So with your numbers -> 1.19125/1.09375 = [b]8.9%[/b] increase.

red numbers are purely for aesthetic purposes

no crit is only for show and does nothing for your damage