Dependent Drop down box similar to vlookup

I need a dropdown box which only shows values under a certain criteria, everything I've tried so far is almost there but not quite, hopefully you'll be able to assist.

My spreadsheet is to display staffing and record if a member of staff is being covered while they are on leave etc.

The dropdown needs to include a pre-determined where the value in another cell on the same row reads certain text, eg "covered". This is similar to a vlookup, but I need all of the corresponding numbers to appear. I have attempted to use the technique on the following link: , however the dropdown then includes the #NUM! where there are no more values matching that criteria.

I also need a dropdown box where additional numbers are added, so if an additional member of staff is brought in to assist with workload, the "post number" is the highest position not recorded, in this example, 9. however my attempt to use IF statements return all of the blank cells as well.

The area that needs work on is highlighted in blue (Column: Post no, second table)

Any assistance would be gratefully accepted.

I've had a look at your attachment, but, it's not quit clear what you want. Can you add to your attachment another sheet of what you want?