You need to put the Target Ratio as expressions, rather than dimensions in order to combine them. Be very careful how you combine the percentages though - as just adding them together and dividing by two will give a duff value. Can you go back to how the targets are derived and work them out in the expression?
My report is a Pivot - if I put the Target Ratio as an expressin it just repeats the figure as a sub-column which is not what I want. I think it must be something in the raw data that is causing it.... I'm going to try that. I take your point about how to work out the % though.... agreed.