Start from the Top down then rework your budget from the bottom up that ultimately creates your final plan.
The best way to set a budget for next year is to look at your historical sales and plan an increase or decrease based on history, if your average growth over 3 years is 8% then set a realistic budget based on 8% increase.
Set 3 overall budgets
1/ High (based on the above set say 12%)
2/ Expected (based on the above set 8%)
3/ Low (based on the above set say 4%)
Every Quarter reset your budget based on Actual Vs Budget reducing the guess work to become more realistic as the year progresses.
Once you have your overall budget then divide it into Departments based on historical sales and if there are no historical sales then its research and estimates that count.
If your average sales increase is 8% then some departments must be above and some must be below to get an average
Divide your plan into months at this point you will have a monthly plan by department per month that will give you a yearly increase of say 8%, then next step is to plan your markdowns, plan your inventory and plan your margin, with an end result of a buying plan, cash flow forecast, profit and loss, balance sheet projection with a complete inventory buying plan.