Systems and methods for determining a bid modifier value include determining, for one or more sets of keywords used in a campaign, a predicted conversion rate for a first device type and a predicted conversion rate for a second device type. Predicted campaign-level conversion rates are determined for the first and second device types using the predicted conversion rates for the one or more sets of keywords used in the campaign. A ratio is also determined between the predicted campaign-level conversion rate for the first device type and the predicted campaign-level conversion rate for the second device type. The ratio may be used as a bid modifier value that relates bids for the first device type to bids for the second device type.