>>>> My dimensions are appearing at the correct distance as a preview
Yes, it is just a preview. You cannot set fixed distanse from measured object to dimension line in dimension style, each time you need to specify this distance manually except for the case when you use continue dimension.

>>>> I'd also very much like to know how to change unannotative or 'Standard' dimensions into Annotative dimensions in autocad for Mac as this could save me time in future.

You can make the whole Dimension Style annotative by checking "Annotative" option on the "Fit" tab of Dimension Style. All dimensions you put after changing Annotative property will get this property.
Or you can make individual dimension annotative by selecting it and enabling "Annotative" property on Properties Palette.