Actually, as you get more and more crit the dmg increase per % gained gets lower. Let me illustrate this: 0%crit to 1% crit = 1% dmg increase. 20%crit to 21% crit =0,833% dmg increase.
This reasoning behind this is: 100 attacks doing 100 dmg with 0% crit = 10.000 dmg, 100 attacks with 1% crit = 10.100 dmg, 100 attacks with 2% crit = 10.200 dmg.
You can see that the dmg increase in % per 1% crit gained gets lower the more crit you gain, so at 20% crit you'd gain a 100/120= 0,8333% dps increase from an extra % of crit.
This math is only regarding the dmg you gain, and not your pet, but I'd assume the value of crit would diminish for your pet as well, given the math posted above.