As presented you offer no opportunity for What If scenarios.

I would prefer to see a set of sliders where the VAT can be dragged to a new value horizontally and the amount vertically.
A presentation like this might work on a phone:
Code:
<form>
    <input type="range" style="width:50%; height: 100%" id="capThou" data-filter="" data-unit="" min="0" max= "9" value="7" ignore="0" step="1"><strong><span id="capThouVal">7</span></strong>,000
    <input type="range" style="width:50%; height: 100%" id="capHun" data-filter="" data-unit="" min="0" max= "9" value="3" ignore="0" step="1"><strong><span id="capHunval">3</span></strong>00
    <input type="range" style="width:50%; height: 100%" id="capTen" data-filter="" data-unit="" min="0" max= "9" value="4" ignore="0" step="1"><strong><span id="capTenVal">4</span></strong>0
    <input type="range" style="width:50%; height: 100%" id="capUnit" data-filter="" data-unit="" min="0" max= "9" value="6" ignore="0" step="1"><strong><span id="capUnitVal">6</span>.<span id="capDecimal">38</span></strong>
</form>
<form>
    <input type="range" style="width:100%; height: 100%" id="capDeci" data-filter="" data-unit="" min="0" max= "99" value="38" ignore="0" step="1">
</form>
Obviously, you would still have an input box for both that can reflect the sliders' setting or adjust the sliders to the typed value.
An on-line example of a slider (almost) in action is http://www.vatcalculator.co.uk.

You need yours to be interactive to steal a march on the large number of simplistic calculators already out there.
This is the type of interactive i would promote: CSS3 Image Filter Generator.xar, which you won't be able to see or use without a Xara Desktop application.
So view here; http://acorn.xara.hosting/ImageFiltering/.

Acorn