Budget Cap should be set with expected CPC and # of SE campaigns in mind

My investigation into the Optus account 80358566000151-0421951541 led to some interesting points that we should probably integrate into our system going forward. The campaign budget cap works well for large budget campaigns, but has the ability to kill low budget campaigns. If we assume an average CPC of around 0.80, a budget cap of 15 days, and 3 SE campaigns per campaign, we can calculate a minimum campaign budget as follows:

N = nominal monthly budget
C = campaign budget cap (in days)

((C/30.4) * N) / 3 = CPC

For C = 15 and CPC = 0.80:

((15/30.4) * N) / 3 = 0.80

(0.4934 * N) / 3 = 0.80

0.1645 * N = 0.80

N = 4.86

Any budgets below this value have the possible effect of being capped at an amount too low to get a click.

We failed to factor in the increase in the number of SE campaigns from 2 to 3 when setting the Optus budget cap. In other markets the budget cap is generally set to 25 days (12.5 per SE campaign). Rather than reducing the cap to 14 or 15 (5 per SE campaign) as we did for Optus, we should probably have increased the cap to 37 to account for the additional mobile campaigns.

Another possibility is to set caps based on the campaign budgets. Generally we use a global setting for the budget cap. We could augment that with account-specific caps for budgets below the calculated min (4.86 for a 15 day global cap).