CRM 2011 JavaScript "Weighted Value" Any help is really appreciated.

Good day,

I've been stuck on this for days now and I can't make any progress.

I'm trying to create a OnChange event to trigger when the OptionSet is changed to calculate a new weighted value i.e.-

value * probability /100 = new read only field

value =                        Currency                     "estimatedamount"
probability=                OptionSet(25/50/75)   "leadqualitycode"
new read only field=   Currency                    "pi_weightedestvalue"

I've created a Web Resource with the following Java...however I'm further away now than I was three days ago.

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function UpdateField() {


est = Xrm.Page.GetAttribute("estimatedamount").getValue();
prob = Xrm.Page.GetAttribute("leadqualitycode").getValue();



if(est.getValue() != null)

{

Xrm.Page.GetAttribute(pi_weightedestvalue).setValue( est * prob * 0.01)


Xrm.Page.getAttribute("pi_weightedestvalue").setSubmitMode("always");

}
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Any help is hugely appreciated.

Regards,

Craig.