First off:

ok now since that is out of the way :) 

June 27: com.tipcalculator_1.0.4.3_a.ipk  the update that adds keyboard support and fixes some decimal issues

com.tipcalculator.ipk < the .ipk tested @ 6:04ish on June 24th on a non rooted pre, works and is deletable. However use this at your own risk as always.

my_tipCalculator_modifications_Source.gz < the source, pretty much the same thing you should get when using ar -x the .ipk

Adding a decimal is a simple task, to do so:

   Download the .ipk

   ar -x the said ipk

 well pretty much follow the instructions on http://predev.wikidot.com/installing-apps-without-rooting until you get the untar'd data file which would have the usr folder.

 #1 under /usr/palm/applications/org.jwz.tipcalculator/app/assistants is firstassistant.js

open the file and add since these are functions i belive their location is irrelevant, however this is Js and i have no solid background here.

  // The dot(.) button
  //
  this.controller.setupWidget('.',   {},
                  {buttonLabel: '.',      disabled:false});
  Mojo.Event.listen(this.controller.get('.'),
                    Mojo.Event.tap, this.dot.bindAsEventListener(this));

// Taps on the "." button
//
FirstAssistant.prototype.dot = function(event){
  var s = '' + this.model.price;
  if (s.length >= 6) return;
  if (s == '0') { s = ''; }
  s += '.';
  this.model.price = s;
  this.update(event);
}

 replace all visible parseInt with parseFloat

also almost at the bottom of the page will be an update section

replace 

  this.controller.get('price').update(Math.ceil(price));
  this.controller.get('tax')  .update(Math.ceil(tax));
  this.controller.get('tip')  .update(Math.ceil(tip));
  this.controller.get('total').update(Math.ceil(total));
  this.controller.get('each') .update(Math.ceil(each));

with

  this.controller.get('price').update(parseFloat(price));
  this.controller.get('tax')  .update(Math.round(tax*100)/100);
  this.controller.get('tip')  .update(Math.round(tip*100)/100);
  this.controller.get('total').update(Math.round(total*100)/100);
  this.controller.get('each') .update(Math.round(each*100)/100);

the reather useless Math.round(each*100)/100) aproach was my quick fix for the endless decimal places.

#2 Here is the second part and that is adding a . button in the UI

which is located at /usr/palm/applications/org.jwz.tipcalculator/app/views/first

scroll down to the bottom and add       <td><div x-mojo-element="Button" id="."></div></td> wherever you feel is appropriate which would make a 3x3 for numbers 1-9 and the last row would be a 1x4

Edit: also changed the icon to incorporate money and coins, also the footer that puts out the suggestive messages is editable at the very bottom of first-scene.html