SA108 entries on disposal of a mixed use property

I have a disposal of a property, where part has always been used for non-residential / commercial purposes and part for residential purposes.  Does anyone have experience of showing this in the SA108.  The property was disposed of as a whole, and so it is necessary to apportion the gain on a just and reasonable basis between residential and commercial.  My initial view is to split the gain and put part in the residential section and part in the other property section.  The other option is to put it all in as commerical and then use box 51 to increase the tax due (ie in respect of the residential part) so that the right amount is calculated as due, but I'd welcome any experience.  I would also add an explanatory note.

thanks in advance.