Welcome to the forum!
Yes, when you scale something using the W or H values in the Selection control bar, it scales up and to the right. If you scale using Object menu > Transform > Scale, it will scale from the center.
But if you want to do it by moving nodes, you can place the nodes to the exact coordinates, by using the Node tool and the Node tool control bar.
Would that work, to move the node, instead of scaling the whole line?
Another thing that might help you with getting sizes precise, is to use Snapping. Have you sorted that out yet? I would probably use a pixel grid and snapping, if I were making clothing patterns. Although I guess nodes will not always fall on whole pixel values.....but if it was patterns only for myself, I'd probably fudge it over to a whole pixel.
I know other people also use Inkscape to draw clothing patterns. But I don't know how they handle the situation you described.