Changing XYZ co-ords by tiny amount

[orbiter]

As I am not a very good artist my models tend to be very Chunky with 90degree edges. I may get clever and have the odd 22.5 degree.

I do a lot of Extrude and then "absolute moves". It means that a face or edge is placed at a known fixed point i.e. the centre of a given face is placed at 5 10 15.
After I have played around with other faces when I come back this is now at 4.9453 10.0034 15.13579. Or some other close but not right values.

I can understand rotating a face by 22.5 degree doing something else and then rotating the first face back by -22.5 degree and it being out by say 0.0001369 as numbers can get rounded.

BUT these are edges/faces I have not touched. If I want an edge at 10.15 and another at 9.85 its because there should be a "wall" 0.30 metres thick. Yet I seem to come back to an edge which now starts at 9.85985985!!!

I do not seem to be able to fix things to 1 or 2 decimal places.

Is it the way I work / think / or a known bug.