As a word processor, LibO workflow is based on margins which define the usable area for text. These margins can be set with distances measured with a ruler on a sheet of paper. Paragraphs can then be offset from these primary margins with indents in Indents & Spacing tab in their style definition. Since we're dealing with continuous text flow (where pages do not exist but are the result of "segmenting" text to the size of paper sheets) vertical positioning cannot be absolutely controlled. Even horizontal positioning is relative to margins, therefore your criticism about calculation.
However, inserted objects (frame, picture, formula, ...) can be positioned absolute in a page. In the frame dialog, Type tab, choose anchor To page and position From left/From top within Entire page. You can now set the top left corner of the frame anywhere using the measurements of your ruler.