basically here is how you do it, use this link for the compound calculator

Compound interest calculator

Enter the start date as the date you received the PPIicon (same for calculating your charges to claim )
The end date is today or whichever day you are doing your calculations.
Tick the box for 360 day year and if you are receiving monthly charges for the ppi as on a store card or credit card then you will need to change the box in the top right hand corner to "12 rests compounded monthly"
You put the interest rate as the same as what they are charging interest to you.
Do this calculation for each payment made, remembering to change your start dates as you go.
Then there is more to do!!

For each payment you now have a new figure! This you need to add 8% interest (this is what you would be awarded by a court )

Use this calculator
simple interest calculator - Blake Lapthorn - Southampton, Oxford, London, Portsmouth, Winchester

You use this in the same way as the other calculator but putting8% as the interest figure.

Now you have your final figure!
Do this for each payment then once they are all done you add them all together!
This is your grand total and the figure you will use for your claim!

Hope this helps.
Obviously this is for cards etc , If you are claiming for ppi on a loan it will most likely be different and easier to calculate as ppi is normally shown on your agreement as a total ammount plus interest.

Regards
LDG