Mix design (agstone) http://www.tree.org/agston.htm
50% hemp fibre for floor (2/3 for wall)
one bucket hemp (10 parts)
3 parts lime (15% of binder)
5 parts sand/clay (25% of binder)
2 parts Portland cement (10% of binder)
Note: mix lime+sand/clay+cement+water first then add hemp.
Putting in a hempcrete floor is a great idea and perfect for making a floor without a DPM (damp proof membrane). We do loads of them here in France, usually with a drainage system on the exterior as you describe.
We usually put down a layer of pumice stones (min. 20 cm) first which acts as a capillary negative/insulation layer, followed by a the first hempcrete slab about 15-20cm thick and then a final hempcrete screed which can incorporate the underfloor heating if its being installed. Some kind of geo-textile layer is generally put down before the final screed goes down to allow expansion movement independent of the lower hempcrete slab. A small trench is dug around the footings, drainage pipe put in a and filled with gravel topped with a geo-textile layer and a final covering of gravel.
The screed can be tiled or polished and protected with a wax finish. The screed can have a pigment added for colour or a coloured wax finish used. The wax finish can be made on the stove using a lump of bees wax dissolved into turpentine.