One more thing...... If you dimension to the lines - as CBL says - before you insert the filets, it seems to be easier and cleaner.
One more thing (hey! I like Colombo...) It is always better practice to dimension to the lines anyway in my opinion - you have probably noticed by now I am not very opinionated, heh, heh

This is really what you are trying to control with the dimension ie. the distance of the line from something, not where its end point happens to be at any given point in time. I put this squarely in the "design intent" concept.
I also like to use minium dimensioning and relationships and use them to force design intent. For example if I am making a square centered on the origin, I will create the rectangle, place a constuction line diagonal, make only enough lines equal to make it square, do a midpoint on the construction line and origin and add one dimension to one line. (That assumes the square will never be anything but a plain square, but you get the picture?) Note also that each line only needs one horizontal/vertical/perpendicular. I try to avoid or get rid of surplus relationships (and dimensions) and let the simple geometry control itself where appropriate. The more excess stuff the system has to compute, the slower things get. It might not seem much until you get to large parts and assemblies. Note that the system does not do this for you. Excess constraints do not necessarily mean an over constrained model. Those errors only (usually) pop up when there are conflicting constraints. But it is often easier to bebug if you get the error and have been careful about keeping down the excess.
John Richards Sr. Mech. Engr.
Rockwell Collins Flight Dynamics
A hobbit's lifestyle sounds rather pleasant...... it's the hairy feet that turn me off.