Freight-In: best way to flow through Inventory & Cogs.

We pay for freight in to our warehouse. The amounts are relatively small vs. our other shipping costs and we have been going directly to the P&L with the items historically, but below the line. I first want to a) make sure we move the costs above the line and b) if we don't simply charge in-period on the basis of materiality, develop a sound basis for expensing over time. I am currently thinking of using inventory turnover as a basis for this (on average 3 months). Our outside accountant proposed % of current month items shipped / inventory. Seems similar but different.