When I did my duke of edinburgh award, I used a 120 litre PLCE british army bergan, and regreted it. I had to much unnessercary kit, but yours sounds much better. Having come to my senses, I now use a lowe alpine 45 litre pack which fits my requirments. I think 45 litres would be sufficient for the amount you are carrying seeing as you are sharing the load. However, if you are planning on doing more hiking, you may want a larger one to stop you from buying 2 packs.