'Waterproof' is a relative term, however the international industry standard is a hydrostatic head of 1000 is rated as waterproof. Most lightweight tent companies use something around 1200 to 3000, which is fine for most users. Going up to 5000 and above might feel nice however I'm a little confused how water gets through a 3000 floor unless you are balancing on one knee while setup in a puddle. A groundsheet won't necessarily help if water gets on top of the sheet and remains trapped between the two materials. You should always think about site selection before setting up your tent, granted this isn't always possible with designated campsites but as a rule you want a well draining site.
10,000 is rated as waterproof
1000 is lightly shower resistant...
the pressure of your body on the floor can throw the pressure up into several thousand, 10,000 is normally enough for waterproof given your body weight is dispersed most of the time. 20,000 is more fullproof.
The ratings are in kilometers of water head. 1000 is 1 meter of water. In your knee description you have about 100 cm2 on the ground if you weigh 100kg (a lot but easy maths) then you have 10kg/sqcm pressure on the floor. A 1sqcm of area with a meter of water above it is 1 litre or 1kg. This means your knee has reached 10000 pressure on the tent floor and a 10000 rating might leak. Anything less certainly will.
In over 5000kms of hiking in three countries I've never had water come through a floor rated between 1500-3000 hydrostatic head. Personally speaking, I like heavy floors for both durability and increased waterproofness, but I've found it unnecessary.
Sorry, geeves, but your maths is a bit askew (although the result is not!).
1. Hydrostatic head (HH) ratings are in mm, not km
2. 100sqcm knees and 100kg would be 1kg/sqcm, not 10kg/sqcm
3. 1sqcm x 1m = 1 x 100 = 100 cubcm which is 0.1L, not 1L (1L = 1000 cubcm)
So, based on the 100sqcm knees, that's 1kg/sqcm = 1L/sqcm = 1000cubcm/sqcm = 1000cm = 10,000 mm HH
you dont need a high HH if the ground under the floor isnt saturated
but if it is saturated which it often is in NZ then you want a 10,000mm HH floor.
overseas tents often arent made for saturated ground and leave the HH low, and rely on an extra "footprint" if people want a waterproof floor
Waynowski, additional 'footprints' are not to add waterproofness to the tent, they are there to protect the floor from abrasion, because damaged floors leak. The idea being you replace the footprint as needed, not your tent. Even with footprints or ground sheets if it rains hard enough and you're set up in flat area or depression, water will get on top of the footprint and become trapped between the waterproof fabrics.