specifying advancing numbers
I am having trouble with calculating the monthly advance of electricity meters. I have used a portal link to separate table. I type in the monthly meter read and this is saved as a record on the separate table, I then enter a new read each month. My problem occurs when I want to work out the advance between the 2 most recent entries. I cant find a way of telling the calculation to look for the most recent and the second most recent entries in the table (and then subtract the second from the first). I am hoping that someone can point me in the right direction...again, it would be much appreciated.
Many thanks for your time,