Column of incrementing values, find difference.

I have a column of incrementing values (electricity meter readings for each day).

For example:

Day 1 - 0001234
Day 2 - 0001899
Day 3 - 0001911
Day 4 - 0002321
Day 5 - 0002391

Every month the spread sheet is renewed, and when I have a full 28, 30 or 31 days of readings, I can take the last reading and subtract the first reading to find the units consumed in the month.

However, I would like to be able to look at the spreadsheet part way through a month and find the total used (as this then fires through some further calcs in the spreadsheet).

Is there a way to look at entry Day 31 in the column, and if it is empty, look at Day 30, if this is empty, look at Day 29 and so on until it finds a value, and then use this, subtracted from the Day 1 figure to find the usage upto the last recording figure?

I think I could use IF commands to achieve this, but it would be intensive! Is there any easier way to this?

Thanks.

Thanks!

Didn't expect a response so quickly to be honest, let alone two, and both perfect working examples!

I have used the LOOKUP method, using numbers rather than text, but allowing leading zeros in the formatting.

Is there away to change the #N/A errors throughout the calculations with a zero instead? Mainly for neatness!

I have attached what I have so far, some of the calcs may appear a little confusing, but it is the columns I am totalling from row 5 to 35, the totals in row 37.

Advice greatly recieved!

Thanks a lot.

File: Metering.xlsx