The suggested ad spend is the amount of money that, according to Odyssey, should have been spent on a particular traffic source. The suggested ad spend is calculated by multiplying the (adjustable) maximum media spend by the attributed revenue.
When the attributed revenue is 500 euros and the max media spend (N.B. excludes organic channels) is 20 %. The sum will be: 500 * 0.2 = 100
Based on these findings you can compare the suggested ad spend with the current ad spend and see what you are over- or underspending.
You can find the ad spend recommendations in the metrics with all the variables and at the cost analysis.