I am trying to set up a worksheet so I can work out how long medication
reduction will take. I need to input a number (the starting dose) and then
input a second number (reduction amount) to reduce the first number by. As
medication is taken twice daily the results need to be spread over two
columns for morning and afternoon.

So far I have days starting in column A8, morning dose in column B8 and
afternoon dose in column C8. I have the starting dose in C4 and reduction
amount in C6. I took an arbitrary value of 100 to start and used =C4/2 as
the first value in column B8 and =C4-B8 as the first value in column C8. I
then used =(C4/2)-C6 to get the next days figure (the starting dose less the
reduction amount). I just referenced previous cells to get remaining days
value. I realise there is probably a much more elegant way to do this
worksheet but I am only a beginner. So far my worksheet looks a bit like
this only it goes down around 110 columns.

Start Total 100
Daily Reduction 2

A B C
8 Day1 50 50
9 Day2 48 50
10 Day3 48 48
11 Day4 46 48

Daily Total 2600 2700

Total 5300

The problem I have is when I input a starting value less than 100 I now get
negative numbers and they are subtracted from the total amount. How do I get
a formula to ignore negative values or better still just return 0 for any
negative numbers. I have sub totalled both morning and afternoon columns and
then added both sub totals to get an over all amount. I thought an if
logical test on the sub totals would be workable but I don't know how to get
the formula to test multiple cells down a column, and/or how do I get each
days figure to ignore negative values. Any ideas?

Glen