Cash Flow Model of Prospects

Hi all,

I would hugely appreciate any advice on this. I am creating a cashflow model (cash in and cash out).
One of the things in the cash out is the different costs at different stages of the process. Lets say there is 10 stages ie intial sales call, meeting, signing contract, getting payment etc. which all have different associated costs
From the initial call, lets say 50% of people then go on to a meeting, and then 10% sign a contract etc etc.
I want to start do a weekly analysis for a year (52 weeks in columns, week 1, 2 etc)
There will be seperate rows with costs for each stage (ie sales call) for each week.
I need to factor in the number of new people at the sales call stage each week (no issue) but also the new people going into the next stages ie 50% to the meeting stage for the next week. This would be no problem if each stage took one week and then the next week has the % of people from the previous stages week. However, I want to have another adjustable input for the time it takes to do each stage in number of days so that i can change it in the future when the exact time becomes clearer.

Any ideas how to do this efficiently would be HUGELY appreciated?

Thanks