If a cell has an entry I need to subtract 5 from it and then keep checking until it gets to the end

Hello everyone.

The Spreadsheet basically is a members list for a monthly lottery at our local church and I need it to do some simple calculations but don't know where to begin.

I need to create a macro to when clicked (on a button or shape)
check if there is a value in multiples of 5 in that cell, if there is then deduct five from it, If not leave it alone and move to the next cell, then check the next cell in the column until it gets to the 500th Number.

can anyone help me with this please.

The sheet is called Members Database and the values are in column R Row 3 and end at row 503.

Having looked in the books I can see there are a load of ways but I am confused as to which is the best.

Thanks in advance