I'm setting up a "biggest loser" spreadsheet for a group. In one column, I'm
putting in the weight at weigh-in each week. In another column, I want to
keep track of how much has been lost from the original weighin.

So, if I have in cell b2 the first/original weighin, b3 I have the next
weeks weighin, b4 the 3rd week weighin and so on to b53 the last week of the
year. In cell c3 I have the formula =b2-b3 to calculate the weight loss.
Now, in cell c4, I put in =b2-b4 to calculate the total loss in 2 weeks from
the original weight. I can keep going down and manually entering this, but I
want to know if there is a way of "filling down" and keeping the first cell
constant (the b2 original weight) while changing the second number subtracted
being that current weeks weight (b3, b4, b5 etc)
I've tried everything and filling down, I then get b3-b4, b4-b5 instead of
b2-b4, b2-b5.

I hope I haven't confused you!