How can I use conditional formatting to mark up this set of error conditions?

Hi all.

I have a lot of data (regarding graffiti) in a table and all my formulas work well. But, because I sometimes make an error in two columns of data I sometimes get results that do not balance. I want to put in a conditional format (?Is there a better way?) that will highlight the error as I enter the data. Can you help please?

Using Excel 2003 (sp3) and no add ons. There are lots of identical rows, 5:400, making up a table. I will use row 10 as an example.

In column M there is a formula which may or may not give a visible result. (It's a measure of the number of days taken to clean the graffiti and will only hold a result if the job has been finished.) The result for "M10" will be a number between 0 and 5 90% of the time, but may creep up to 30. It may also leave the cell looking blank although the formula itself is still lurking out of sight.

A matching row of cells, O10:T10, have no formulas in them. I must manually enter a "w" or an "a" into ONE of these columns, depending on the graffiti location (Worthing or Adur) and property ownership (Columns for residential, commercial, council etc.)

So now the errors I need to flag up as I enter data are to ensure that M and O:T balance. There should always be a result in M if there is an entry somewhere in O:T and if there is an entry in M there must be an entry somewhere in O:T.

Sometimes I mistakenly put an entry in O:T even if there was no result in M. Sometimes there is a result in M and I fail to put an entry in O:T. I need to be alerted as this happens, if possible. It will save hours of trying to track down my data entry errors.

BUT, there is already one conditional format already in M. It highlights the cell if it goes over 5 days for cleaning. =ISNUMBER(M10)*(M10>5).

If you can help, thank you very much. I thank you, my wife thanks you for me not doing overtime trying to track down my errors, my boss's thank you for making me more efficient and other forum readers may thank you for helping them to improve their sheets.