Two Knockout computed dependent on each other
I will review and contrast the SO answer that I liked best: 

function viewModel() {
    var self = this;

    self.NetPrice = ko.observable(100);

    self.TaxRate = 0.2;

    self.TaxAmt = ko.computed(function() {
        return parseFloat(self.NetPrice()) * self.TaxRate;
    });

    self.Total = ko.computed({
        read: function() { 
               return parseFloat(self.NetPrice()) + self.TaxAmt();
        },
        write: function(val){
                var total = parseFloat(val);
                var taxAmt = total - total / (1 + self.TaxRate);     
                self.NetPrice(total - taxAmt);
        }
    });
}



It is more standard to create a constructor, then to create an object with object notation and then add functions.
Not to big a fan of using self everywhere instead of this in the SO answer
Do not use console.log in production code
Do not use calc as part of the variable name
Do not use a useNetPrice type of indicator, knockout can figure out the computed values on its own and knows when to recalculate what
write functions do not have to return anything