Commercially available tax calculators/spreadsheets

......to help me determine if incorporation is worthwhile.

I have had a few unexpected results recently where the business use of a car has impacted on the calculations and I would like to have a spreadsheet with which I can check my calculations easily.

The spreadsheet would have to be fairly comprehensive and cover total mileage and business mileage as well as the original cost of the car. e.g. one client likes expensive cars (c £40 - £45K) but only does a total of about 8,000 miles a year of which about 6,000 of those are on business. By converting to a limited company and claiming the 45p/mile he would only receive c £2,700 p.a. but as a sole trader he receives 75% of his motor expenses and through capital allowances, the cost of his car - a significantly higher figure.