14-15 knots. Although learning with a 7.0 will mean a lot of effort swimming the sail into position in the water. Ideally a 6.0 in 17-18 knots would be easier. Remember you can practice the technique in 4 foot of water before moving to deep water.
..but I tend to think one can learn sitting on the water the height of the board plus the fin lying down and work from there at least that's how I learned..and one more thing just go to the water and do nothing else besides this..it'll be hard though when there are sailors zipping by..
^^^Agreed 
I dedicated a whole day to just waterstarting.
A good drill to do is once the rig has lifted you up and you're pushing forward, just drag your front leg through the water for a while. This will help you obtain that balance needed. Worked for me.
The hardest part for me was when the clew sinks. I just swim the board and rig into the breeze and when you lift the sail out of the water, lift it up and forwards/over your head. This should get the wind under the sail and the clew free from the water.
I've taught a few people to waterstart. One of the common mistakes to look our for is having your font hand to close to the mast, which means you usually round up as soon as you start to get lift. Moving the front arm back gives you more more power and less chance of rounding up when you are coming up on to the board.
I think a lot of people try to learn in too little wind. You need to be able to plane comfortably. Otherwise you need to use too much leg action treading water and get tired, or use too much legs to stand up on the board rather then being lifted (which means rounding up due to too much back foot weight)
I think best time to learn is on the days where it is hard to NOT to get lifted out of the water as it is windy
Beach start and work deeper. Back foot first, try not to do both feet at once when waterstarting as its a bad habit and slower in the long run (it does work and when very windy you will do it that way probably but in lighter winds you will see people lay back waiting with both feet on the board for the wind to do the work....it wont).
learn where the balance point is, where the sail will hold you body weight bouyant in the water without lifting you up, you then rotate the tip of the mast into the wind a bit while getting up over your back leg and kicking (like treading water) with your front (which is still under you in the water). If you do that bit too quickly or rotate the mast too much it will end up being ripped out of your hands and end up downwind of the board......keep the uphaul as its easy to just get on the board and uphaul when that happens......
While beach starting get used to flipping the rig while standing in one spot, you need this skill for rig recovery.
Waterstarting is really easy, rig recovery takes a bit longer to learn. You will get good at waterstarting and then for a while lose all your energy recovering the rig in deeper choppy water. Common sense here is a must, learn to let the wind do all the work for you, and if it gets difficult then move to the tip of the mast, rest while the wind blows the board downwind and you are in a natural position to clear the rig for the water. Avoid sinking the clew at all times, if you do then swim the mast into the wind or use the board to help clear it.
To flip the rig you feet are used as anchors/pivot points in the water, when you get there you will see what I mean.
Sorry if this is a bit rushed, its bed time over here in the uk, and it was a long session in cold water today....first for 3 weeks, longest time off the water for 6 months
The single, best piece of advice I could give anyone on waterstarting, especially in marginal conditions, is to pump the sail upwards once it is in position.
This exposes more of the sail to the wind, and generally gets you on the board quicker than waiting for a gust.
I only sail with smaller sails (4.2-5.7m) than mentioned in the first post, so I'm not sure whether this applies equally to the larger slalom/speed sails (7.0m+)
Cheers
