Isoplane dimensioning

Hi

Do any of you produce either isometric or pictoral style drawings where you want to place a dimension (in the drawing doc) on a plane contained within the part or assy?

Ive attached an example here but cant find out what it is that determines the plane that the dim will be on.

The dims in the example started to fall where I wanted them purely by luck, now I want to place some others on another plane and want to know how.

All help appreicated

Thanks

Darren

SolidworksDrawings And Detailing