Scale in layout question

[Ands43]

Working on a imac.

I have inserted my image - right click - scale - then there are options such as 1:50, 1:200, 1:1000.

Is there a way of specifying my own scale say 1:75 or 1:33.3. When I grab the blue frame there are measurements changing in the bottom right hand corner, if I go to alter these by moving my arrow away from the blue frame the measurements lock so I can't type in the box. Does that make sense.

This is an issue for me as I'm being asked to do plan and elevation drawings in sketchup.

Thanks

[JuanV.Soler]

I run windows, but maybe the same,
if you click under Layout preferences you can custom your own scale

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[Ands43]

gracias!!! so simple!!