All Umbrella Companies are controlled by the Inland Revenue and use an identical calculation to work out your tax. The tax you pay is dependent on:
1) Your personal tax code
2) National Insurance contributions, (deducted at a fixed percentage)
3) Expenses, (you can only claim those expenses that the Inland Revenue approve)
Remember, a dispensation means that the Umbrella Company does not need to see your receipts; it does not mean that you don't have to have them! The only factor that will affect your take home pay is the Umbrella Company's fees.