Please see video linked below. The selected nodes are precisely placed and I use the shift arrow keys to increase the values of the X in units of 10. The lock icon is enabled to keep the proportions scaled, in this case 1:1 but scaling the item up and down in units of 10 results in the proportions changing.
The video is here. I am using Glyphs Version 2.6.5 (1342)
https://we.tl/t-JAxpaFbhMr