If you want to set Lineweight to ByLayer, you should set DXF 370 to -1...
If you want ByBlock => (370 . -2)
If you want Default => (370 . -3)
And if you want millimeters => (370 . x) and x=(0 5 9 13 15 18 20 25 30 35 40 50 53 60 70 80 90 100 106 120 140 158 200 211)
Note - once added DXF 370, can't be removed like '(66 . 1) - block with attributes, so if not present I'd leave DXF data unchanged, but if (assoc 370 (entget ent)) => (entupd (cdr (assoc -1 (entmod (subst '(370 . x) (assoc 370 (entget ent)) (entget ent))))))
x - you'll have to choose...
[EDIT : I was wrong for my note... I've changed lineweight to some value, then (entupd (cdr (assoc -1 (entmod (subst '(370 . -1) (assoc 370 (entget ent)) (entget ent)))))) and miraculously DXF 370 disappeared...]