There is no specific tool for this task in LibreCAD I think.
When you say combination of different arcs, do you mean elliptical arcs or spline curves?
Then I think the only option is doing some math, compute the point on the arc and use the command line for exact coordinate input.
For arcs in the sense of circle segments, there are some options that may satisfy your needs.
When you select entities, there is a status box at the bottom, where you can see the number of selected items and their total length.
Then there is the List entities plugin, which gives you details like radius, start and end angle of selected items.
Now you need to know, that a specific length of an arc can be expressed as an angle too. The circumference (2 r Pi) of the circle is equivalent to 360°. Using the rule of three, you can compute the angle that is equivalent to you specific length.
Knowing the angle you can construct two lines, one from the arc center to its starting point and the second starting in the arc center too, rotated by the needed angle from first line. The intersection of the arc and line two is the point you are looking for. Use the snap tools for this construction to get exact positions of center and starting point. This method lets the arc untouched.
Another way is the property editor, find it on the Modify tool bar and click on an arc. In the Geometry box of the property editor you can modify Start and End Angles to match your computed angle. This option modifies the arc and will shorten or lengthen it.
thank you very much for your answer! It will work this way.
Maybe you are interested with what you helped me. I started a small Fair Trade business. I imported Fair trade fabric, made patterns, and sewed shirts. Now the production will be done in India, where the workers need a fair salary. Therefore I need to digitize the patterns. This is usually done with special pattern programs, that are more expensive than I can afford. (CAD Assyst 10.000€) So I learned how to work with Librecad (a little). Last step are the notches. These are marks in the fabric, so the workers know that the fabric is laying in exact position, when sewing. If you have a look at your shirt, you notice that the arm hole and the sleeve are curved in opposite directions. On these curves the notches have to match. So a function of departing from a point for a given distance on the arc would be the easiest. But the way you suggested will work as well. Thank you again, you've just helped to save the world:)