Conversion rate is the ratio of conversions to clicks (you can multiply by 100 to state as a percentage):

conv_rate = convs / clks

Or:

conv_rate_t = (convs_1 + convs_2 ... + convs_n) / (clks_1 + clks_2 ... + clks_n)

No need to calculate a weighted average for this one. But if the number of conversions wasn't handy, we could rearrange the terms in the first expression:

convs = conv_rate * clks

And plug into the second to get:

conv_rate_t = (conv_rate_1 * clks_1 + conv_rate_2 * clks_2 ... + conv_rate_n * clks_n) / (clks_1 + clks_2 ... + clks_n)

-- Brian Kennish, AdWords API Team