It's sound calculation (happy to share my Google Sheet with you if you want).
Based on the May 2016 prize distribution, the £25 prizes form 83.89% of the total prize pot so everything hinges on them and they are the most numerous.
You *should* be taking 1-2 £25 prizes per month (May 2016) - an average of £325 per annum. What were you getting last year?