That's going to overstate the resulting avg. crit. by a bit because during the proc you will spend slightly less time at +920 crit and more at +184. I modelled this, and it turned out to about 3.5 crit. So add a fudge factor of -3.5 to the formula if you like.
Note: this will set up a circular dependency in your spreadsheet if the NIC is equipped: the calculated crit value depends on the character's crit rate which depends on the NIC's calculated crit. You can solve this short term by copying the NIC formula, putting some bogus constant in, then copying the formula back. It can probably also be solved by fiddling with circular formula resolution.