Set Tax Value on order Form

I am trying to set the total tax amount on the order form using jscript. I am able to put a value into the field but when i save/recalculate the tax amount does not stick. it gets over written with 0.00. I think this may be because there is no value in the Order Products tax field.

I have written a clunky script to help me see why I this is:

/// <reference path="XrmPageTemplate.js" />
function OWS_Library_calcTax() {
    var ows_formType = Xrm.Page.ui.getFormType();
    var ows_taxBase = 0.0875;
    alert('OWS Form Type ' + ows_formType + 'Tax Base ' + ows_taxBase);
    var ows_state = Xrm.Page.data.entity.attributes.get("shipto_stateorprovince").getValue();
    var ows_orderTotal = Xrm.Page.getAttribute("totalamountlessfreight").getValue();
    var ows_orderTax = ows_orderTotal * ows_taxBase;
    if (ows_orderTotal > 0) {
        alert('Tax total ' + ows_orderTax);
        alert('Greater Than 0 so display OWS Order Total ' + ows_orderTotal);
        alert('pre tax State is ' + ows_state);
    }
    switch (ows_state) {
        case "california":
            alert("california tax "+ows_orderTax);
            Xrm.Page.getAttribute("totaltax").setValue(ows_orderTax);
            break;
        default:
            alert("Not California of any kind");
    }
}

I think I could get around this by applying the tax value in the order products section, but I am not sure how I can get the order shipto/billto state as each product also has an address attached to it. Anyone have any ideas?