Dan you are correct, the ratioplot will only give a visual of the peaks when displayed as a ratio, and not peak area.If you want that, you need a peak real cf and include the channel names in your formula. For instance, derive 2 channels with the fixed wavelengths and call them 250nm and 210nm. Then your cf formula is U%.%.250nm(Area)/U%.%.210nm(Area) and this will display the peak area ratio from the 2 wavelengths presuming your unknown samples are labelled as U...etc.
Or you could try MarcNoble idea.
Are you using a PDA? If so, In the method set used to acquire the data, create a derived channel called Ratioplot. Open your method set, right click on Derived Channel< New Derived Channel and pick Ratioplot, pick your Numerator wavelength, your denominator wavelength and a minimum value, call the derived channel something like Ratio and assign a processing method beside it when it appears in the Channels table.
Save all changes, open up some raw data, apply method set and in the 2D channels tab, your Ratio should be populated with each peak value of 1 wavelength divided by another.