Need number formatting help...

I'm working with numbers around millions. Precision isn't quite necessary, so If a cell has a number larger than 999'999 want formatting show me a rounded up version of the number say if its 15'435'245 i should see, 15.4m ... But still have the cell contain the full number.
Also I'd prefer to keep the thousands separator if the number is less than a million.

I can do that in a cell with :

=IF(A1>1000000,ROUNDUP(A1/1000000,2)&"m",A1), but then i cant refer to that cell for calculations.

... I've experimented with the syntax in the custom formatting option, but apparently I don't know enough to make it work.