plot an uptake curve into cells automatically

I would suggest that this first might need to be a math problem before we make it an Excel problem.

Let ti be time (in weeks, if that is easiest). so t1=end of week 1, t2=end of week 2, and so on.
f(ti)=??? This function could be any function you can imagine. What kind of function seems reasonable? Something that grow exponentially - then become constant when the plateau value is reached? Something that approaches the plateau value more asymptotically? Something that simply grows linearly until the plateau value is reached? Other relationship?
Then, you simply need to adjust the parameters for f(ti) until sum for i=1 to 74 of f(ti) = 850 (or whatever your total is for this problem).

Not very detailed, I know, but much of the details are going to be inside of what you choose for f(ti), and there are essentially an infinite number of available choices for f(ti). Based on what I see in your question, this will be the first part of the question to answer -- what sort of function do you want to use for f(ti)? If you need some ideas for some basic function types, heres a quick refresher on basic function types: http://mathonweb.com/help_ebook/html/functions_4.htm