Help with Expression in UNION Query (1 Viewer)

Please help.

I have the following:
tblBlotter
qryBlotter1
qryBlotter2
qryBlotterCounts (UNION query)

Two of the queries are alike - this way I can get the data I need for past months history. The third query just UNIONs the other two together.

I just wanted to add one more expression at the end of the query called "qryBlotterCounts" to complete it.

What I need is- if this is Month2, I need
MonthlyChange=((Month2-Month1)/Month1*100)
and
MonthlyChange=((Month3-Month2)/Month2*100)

Always... No matter what the current month is.

I'm just not sure where to begin, given I now have 3 queries :/
I have attached a sample DB.

Suggest you dont open multiple threads on the same subject, in particular without linking the previous thread.

http://www.access-programmers.co.uk/forums/showthread.php?p=1330184#post1330184

http://www.access-programmers.co.uk/forums/showpost.php?p=1330347&postcount=22

thanks for your help!