TL;DR
Allow specifying the center of the blueprint by entering an (x, y) offset instead of moving it with the mouse.What ?
For the grid size there are two integer boxes to modify the size. Add two such boxes for the center of the blueprint as well.Why ?
For large blueprints moving the center to the exact tile can be difficult. Some people aren't as fine motoric as others and will jump between one to high and one to low for ages and tetsing that the alignment fits is a painfull process of opening and closing the blueprint all the time. If after testing one sees the blueprint is 1m to much to the right one can simple edit the center by one. Doing so with the mouse is painfull and one easily overshoots on large blueprints.Also sometimes the center of the blueprint should be outside the blueprint and one can't drag the center that far.
As an example I tried making blueprints for a city grid rail design. One large blueprint that contains a whole block (4 crossroads with straight rails between them in a square). And then blueprints to add train stations to the sides of a block. They are as wide as a block but not verry high. But their center should be the center of a block so they can be rotated to fit any of the 4 sides. This simply doesn't seem to be possible to achieve without adding a dummy item to make the blueprint as large as the block.
