As a rule of thumb I allow 4 to 5m² per 20kg bag of adhesive, yes I appreciate that there are other variables like substrate, Tile size, Trowel size etc but 4 to 5 is a good average. As for Toppfix adhesive, I wouldn't touch it with a barge pole - personal experience with this adhesive has shown its quality and consistency to be too variable. Mapei Adhesives are very good and the new adhesives at Tilegiant - Proquick and Pro Flexible are very good value and are made by Mapei.