pure helium is hellishly expensive and you wont find anyone using it as I understand none of it is produced in Australia. So what we mostly use is party/balloon gas which is helium mixed with nitrogen. Some guys have access to hydrogen but this is not recommended as it is explosive. Anyway, how much you fill the balloon is dependent on the weight of your bait and what fish you're after. Skipping the surface means you're after spaniards and if so then it is correct. 

 Rig up put bait on fill the balloon  to what you think would skip it send it out ...then adjust accordingly by letting gas out or putting more in. It also depends on how strong the wind is after a few goes you should be able to get pretty close to what you need

 its a game depending on bait bait and wind. I have sent one in and out about 6 times to get it skipping right. 

The more the setup is let out will control the height.If the wind starts to drop you just wind it in till its right again

 PM me your number mate and I'll call ya talk ya thru it

nowhere near as technical as it may seem when you're sitting at home trying to image how it all works

ive heard of people doing the math on lift / volume of helium verses weight of bait and rig etc etc....all this is really all rubbish because of the variables you can't control like wind and height of cliffs or beach etc etc....plus wind gusts and the more line you have out the more gas you need cause of weight of line 

As ChrisG says there are so many variables that it is impossible to have an exact formula. There are general rules like more wind = less gas and obviously then less wind = more gas, more distance = more gas (line drag (less drag for braid more for mono)). One thing to remember with any starting point though is that it doesn't cost you anything to put more helium in but it costs money to let some out!! The only time I ever mix air with it is to increase balloon size and wind capture while limiting lift if the wind is sub 8km/hr. Other than that using pure helium will maximise the "lifting life" of the helium if you are using the balloon again the next day as an air mix will degrade it much quicker. From my experience the best bait size to use is around 300gm. Obviously the lighter you go the less gas you use but it gets very tricky with lighter weights if the wind is inconsistent as you will be up in the air in no time. I have used 30cm chopper tailor as well which need a lot more helium but they work a treat and you can get a very good action with lots of splash on the surface in windy conditions. Also don't forget to put a bright skirt on it too. Have fun and I hope you have good success!!

thanks gents appreciate the advice!!

Another question if anyone can help, how do you tie your 90cm balloons to the leader so that you are able to untie it easily to readjust size of balloon when needed? the clips I have seen are too small.

I usually tie the balloon on using a Uni knot around the neck of the balloon and use it as a noose. You can wiggle the Uni knot back to let air in or out and then tighten it back up when your done.

  Yep!