brotherton-erpnext/erpnext/docs/user/manual/en/setting-up/calculate-incentive-for-sales-team.md

Can be used in any Sales Transaction with Sales Team Table:

cur_frm.cscript.custom_validate = function(doc) {
    // calculate incentives for each person on the deal
    total_incentive = 0
    $.each(wn.model.get("Sales Team", {parent:doc.name}), function(i, d) {

        // calculate incentive
        var incentive_percent = 2;
        if(doc.grand_total > 400) incentive_percent = 4;

        // actual incentive
        d.incentives = flt(doc.grand_total) * incentive_percent / 100;
        total_incentive += flt(d.incentives)
    });

    doc.total_incentive = total_incentive;
}