That gives you weighted contributed rake - which is already in the column ant_rake_weighted_contributed (or amt_rake_weighted_contributed_curr_conv). To get 15% of that value, multiply by .15 - if you're just going to use these values directly you don't need new columns, just use the rake columns and multiply by .15 in your stat.
You can download custom statistics for rakeback from our Download Warehouse. What statistic you need depends on what type of rakeback deal you have but if there isn't a matching one you can edit the values to suit your own requirements.