I would assume you calculate how long it takes 1Khash/s to solve a share of difficulty 1:

which is 7 weeks, 17 hours, 2 minutes, and 47 seconds on average

convert that into seconds:

4294967 seconds

so lets say you have a card hashing at 350000 khash/s, that well submit a share on average, every 12 seconds.

4294967/12= 358163.91 Khash/s

I'm off by 8 Mhash/s because of the rounding in the math for the average rate of solving a share of difficulty 1. More precise calculations can be had be had by using this formula and rounding less:

1/((2^224-1)/(share difficulty)*(hashrate in khash/s)*1000/2^256)

So you'd get a number around 12.27. You can also solve it backwards. 4294967/x = hashrate, then solve for x.

Ex.

so you simply would look at the number of shares over a period of time. Let's say we had a worker submit 360 shares over an hour.

360 shares per hour is 6 shares per minute, or 0.1 shares per second. Bring that up to a whole share, 1 share per 10 seconds.

4294967/10 = 429496.7 Khash/s

Tada!

1PRieLgL24cr39sWsNYUKbLJ3kWuBHPJBA if you feel so inclined