How many different scaled viewports do you have?
If you only have one, then you can setup you ltscale to be what you want, and then set psltscale to 0.
If you have more than one, then it is a little different. You will have to set you ltscale to what you like, be it half the dim scale in model space, and then when you switch to paper space, you will want psltscale set to 1, and then set ltscale to whatever the equaltion you can up with in model space. So if you are drawing a drawing, and it will be on a sheet at a scale of 1" = 1'-0", your ltscale in model space is 48, and your ltscale in paper space will be .5, with the psltscale set to 1.
Hope that is clear.