I did a 12 inch round on Friday and was supposed to use nearly 2kilos of marzipan - I only ended up using about 1.5 and that's with an extra layer on the top of the cake as I prefer to use about half the thickness around the sides and ice using one big sheet then attach another disc.
A lot of recipes seem to work on the 1/2 inch of marzipan, 1/2 inch of icing principle which IMO is far too much.
I will do my icing (regal, not sugar paste!) on either Friday or Monday next week and I tend to spriyz with water rather than using sieved apricot jam which is what I used to attach the icing.