Agresso FA Module - treatment of a Disposal


I'm fairly new to the world of Capital Accounting especially using the FA module in Agresso. Just looking for some advice, and it may not be linked to the use of Agresso, just the basics of CA!

If we have disposed of an Asset, 100%, how should I be working out the P/L.

The Land is worth £60k, the Building NBV £45k. We rec'd £70k sale proceeds - so how much of the £70k is attributable to the land and how much to the Facility? We usually use a 80/20 rule of valuing, so is it the same?