comission + premium charge

Am I correct in saying that if you were to have a gross profit of £100 in a week (when already a pc payer)
then the maximum combined charge of basic comission plus premium charge would be only £20 ?

or £200 on a profit of £1000 etc

have just emailed regarding what to me appears an error this week but seems to have been over 20% the previous one as well -
Am I missing a subtlety here ?
have to admit I've rarely bothered check the exact % in recent times but looking at an old spreadsheet they all seem to work out correctly @ the 20% mark

Any help or ideas appreciated