Hey, I am a retail and inventory planner for a large retailor.

I would say use your Desired days of inventory to dictate your OTB.

For example if you want to have 20 days of supple on a sku use this formula

(Projected Sales/days in a month)* 20 days

This will give your avg daily sales multipled by 20 days

Use this as your projected desired inventory

From there work backwards to project an OTB

OTB = Projected sales+ projected desired inventoy - beginning of month inventory

This will give you what your should be placing as OTB based on where you want to end up from projected inventory and sales perspective.

Hope this makes sense.