I have just seen your post, so I apologise for jumping in a bit lateer on.
In your example I think you are confusing the rate of pay with the actual holiday entitlement of your client.
As Alex says you need to use the Holiday entitlement calculator on ther Gov.UK website to work out someones actual holiday entitlement, if it is less than the minimum of 5.6 weeks. Part timers get a pro rata entitlement of the 5.6 weeks.
So if your client, using your example, works 303 hours per year then this is 5.83 hours per week X 5.6 =32.65 hours annual holiday entitlement. Then if the hourly pay rate is £7.50, then the annual holiday pay is £244.88. So not hours as you stated.
Then if your client works an average of 5.83 hours per week and then takes a full week's holiday then the Holiday pay is 5.83 X £7.50 (hourly pay rate) give the holiday pay amouint as £43.73. So not your figure of £4.71.
I hope this makes sense.