How can I create a text box that will return a single?

I have user form that has two labels and one text box. Label One shows the
base rate. Text Box allows the user input a markup. Label Two should show the
total rate, consisting of the markup and the base rate.

I programmed a label.caption update at the text box change event, but I get
a data mismatch because the text box returns text even if the input is
numeric.

Also, how can I configure the text box so that all values input show as
percentages, similar to the % format on a normal worksheet cell?

Thanks,

Paul

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Paul