Either my math is off or yours is. Gross profit margin is (to my understanding) a measure calculated by dividing gross profit by net sales.

In your example, with net sale of £109.24 and cost of goods at £65.81 generates £43.43 gross profit. Using the definition above, 43.43/109.24=40% According to MY calculation, your selling price would need to be £124.17 to return 47% GP.

To calculate this number, the formula is =cost/(1-margin). In your example, this would be £65.81/(1-0.47) or £65.81/.53=£124.17

If I am missing something here, please let me know.

Cheers!