Great question @wk. You can edit the dimension to be exactly 2.5 degrees but at the moment you cannot set the number of decimal places to display. It is a good suggestion for an enhancement.
@TJBuchanan is desigining some improvements related to this and may have some additional thoughts on the topic...
Its great to hear from you again on this topic.
The scale in Catchbook is set by creating 1 single dimension and then editing its value. When you do this the geometry will not change, but the scale is updated. (This wil not happen if you have 2 or more dimensions on the page!) Any subsequent dimensions you create will be created at that scale.
So...if you edit your 1st dimension to something a lot smaller than the default value it will have the knock-on effect of showing all new dimensions with more decimal places.
I have seen an issue where the 1st dimension does not include more decimal places after the intial edit. Perhaps this is the problem you are seeing?
You will be able to manually control the number of decimal places that show on each dimension in the next update.
Hope this helps.