Begin with $50 per day (or an amount that makes sense to you). Try to split the Smart Shopping campaigns to two different geographical locations, then allocate the ad spend to each campaign based on performance.
So two identical smart shopping campaigns.
1: All of (ex: Country), excluding (ex: State) 2: (State) while excluding the rest of (Country)
Whatever ends up being limited by budget, or having the best ROAS, gets the most ad spend.
Since they both will have their own dedicated daily budget, you will want to balance that on how each of the campaigns performs.