Don't. It just does not work modelling historical divisions with the hoi4 system.
You end Up with either wrong Equipment count, wrong head count, wrong number of units or very poorly performing designs. Non of this satisfies my inner bean counter.
Since the AI will spam as many Units as it can, you cannot afford a historical OOB, since AI does not give a damn about historical numbers.
Missing Independent units is another pain.
I dropped historical division design and took a "plausible If reality was hoi4" approach.
My 1939 Inf division as Germany had 3*4 Inf battalions, 1AA, 1AT, 2ART, Eng, Rec, Hos, Sig
Panzer divisions end Up with 8Larm/marm mixed, 5 mot/MEC mixed, 2 mot ART and 1 AA/AT mot each. Same support as above plus maintenance.
My life has improved so much!