do you have a question in general how to build a map in QlikView or how to show only customers closest to a City of choice? So user will select a City in listbox and then you want to see on a map that city and customers within 30 miles?
Are going to use a scatter chart with background image technique for your map or any extensions? I would do a scatter chart and put x and y expressions with outer IF statement like that:
if( distance>30, customer_lat)
if( distance>30, customer_lon)
where distance would be your formula above. so the idea is to show a point on scatter chart, only if that customer lat/lon within 30 miles of a selected city