That is a problem. You can go back and forth a lot. You can turn on grid and take carefull notes of how many inches you need. You could also, perhaps, bone the strings and pose them manually. Would probably take more work to get the bones adjusted right than to go back and forth tweaking, tho. Your instrument looks to be similar to a medieval lyre, somewhat like the 7th-centurt rote uncovered at Sutton Hoo. I look forward to seeing how it comes out.