Identifying cell value based on date function

[2013A4MT]

Hi all,
First post, so apologies if not the clearest request.
I'm a fitness coach looking to track my client's daily weights and keep track of how many pounds they have left to lose by a certain date. I'm struggling with finding a code/function that will enter their current weight into a cell, and use that value to calculate pounds left to their goal weight.
I'm not sure if this needs to be part of an automatic calendar or not. I've included the basic worksheet.
Thoughts and advice appreciated.

[cebalaw]

What values are expected to be entered in rows 7 and 8, columns B to AE?

[FDibbins]

I think it would be helpful if you included some sample data, and show what you expect to see - even if you have to enter values manually.