calculate average percenrage

im trying to calculate the Average achievment percentage by all sales agent. i have created the achievemnt percentage with a measure: calculate(sum(achievmenet)/sum(monthly target)) and i need to get the average achievment for all agents each month for each channel excluding any agent wih position (retail sales supervisor) . in this case the average achievemnt % should be calculated for all the 9 agents with position (retail sales agent) for the selected channel 'Mahboula' and month =' december2022'