i wouldnt be too concerned what money they are going to let you play with. From my experience you start off trading one lots as your clip size with maybe a max risk position of 4 lots. you wouldnt need much cash to trade that size on your own prob about 3/4K. but as you get better you trade larger so if you were backing yourself you would need more money as margin. You could be trading a coupla hundred lots as a backed trader. The thing you would be interested in is your percentage of profits, and your round turn costs. And these will change as you get better because you have a track record to bargain with. And in the end you may want and be confident enough to be totally 100% backed by yourself, but if you blow up its your cash so there are different pressures.
The place you work at will have plenty of margin available, and they will be giving it to the people who are making money and letting you trade bigger, because it is in their interests as they are getting a percentage of your profits and a slice of your RT costs (larger size =more RT=more money for them)
Plus its not how much money they let you play with its how much money you make with it, there are guys out there who can trade a smaller size but make far more money than the guy they aresat next to. It can get a bit too much of an ego thing the size you trade. So dont get to caught up in it
the rate of return they will want is for you too cover your trading costs (not desk fees)as quickly as possible, so if ur trading on 1 lots and ur costs are either 1 pund/euro per trade and each tick you make gives you 10 euros/pounds then to cover your RT you need to make a tick every ten trades ie 10% hit rate. The next step is to cover your desk fees so your hit rate will have to be above 10% and the difference will be your profit which hopefully pays your desk fees and leaves you some cash. As you get better your costs can decrease to say 6% so your profit gets larger without you changing your trading style. So its hard in the beginning but once you break through it becomes much easier. Depending on your deal (firm)you may have no desk fees in the begining or at least subsidised which helps as it gives you breathing space.
hope that helps you