his critical strikes are... odd, since it doesn't work like a regular crit.
Regular crits add 100% AD damage to an autoattack, 150% AD if you have
but in Graves' case instead of doubling his autoattack damage and shooting 4 pellets, he shoots 6 pellets in a wider cone and all of them have a bonus damage of 40% x (1 + bonus critical dmg) AD. You can read more about that on the Wiki: http://leagueoflegends.wikia.com/wiki/Graves
The thing with Graves is that if you hit more than one pellet, then your damage goes through the roof! so he needs to be in close range in order to maximize his damage potential, for consistency's sake, let's do a fairly easy calculation:
with a
you get 150% critical damage and 70 AD, let's bring the AD value to 100 so it's easier to calculate things, in that case each pellet would do:
Formula: 40%AD x (1 + bonus crit dmg)
40 x (1 + 1.5) = 40 x 2.5 = 100
In this case a crit adds 100 bonus damage to each pellet
the first pellet that hits has a maximum value of 100%, so the first pellet hit does: 200 damage
Subsequent pellets are only 33% effective, so each one does: 133 damage.
Given the case that you hit all 6 pellets then: 133 x 5 = 665 + 200 (from first pellet) = 865 damage
Of course, hitting all 6 pellets is pretty risky and almost impossible to be honest since the cone widens up, but theoretically it's possible.