hi everyone, first post on the forum and would really appreciate some help with a problem....

I'm a manager of a charity that has customers (service users) who use the service for varying periods of time, ususally between 3 and 12 months (the length of the program of support). New customers enter the service as time progresses, which can be measured monthly, so for example we might have 50 referrals to the service each month.

I've tried to create a 'calculator' in excel to enable me to work out the answer to the following question...i've attached it but im not convinced its all that accurate (boxes in white can have figures inputted, boxes in beige output figures) here's the main question i need to answer...

1. If x number of customers commence services each month, and the average stay with the service is y number of months, what will the average number of people using the service at any one point be?

This will help me work out how many staff i would need to provide the service if x number of people are referred to it per month, and stay for y length of time.

i might be complicating the issue by trying to incorporate the total number of customers at the start of the period...

Anyway, if anyone has a few moments and wouldnt mind taking a look at what i've done and can show me where i might be going wrong, that would be great .

Thanks in advance! Mike