UK Internal Recharges

[Kaite]

I was wondering if anyone has ever used any formulaes to calculate internal recharges? I need to allocate one of the area's cost to a different department and got stuck with the allocation. I tried to take a ratio of each department's cost to a total organisational cost and using this percentage apply the recharge proportion. However, I was wondering, if there are any more ways to find better solution and what logic could be followed. Any advice is greatly appreciated.