I would never ask for any advance payments, i think it's unproffesional & i tend not to trust trades who ask for payments in advance to buy materials. If you have done first fix and agree with the client in advance, then i see no problem with that. I personally dont ask for any payment until the job is complete. Most of the time the client is trying to pay me before the job is complete. One piece of advice i was given from a friend 18 years ago, when i started in business was, you must put aside some money and always keep it seperate for stock & eventualities. This has always served me well and i have transfered this over to my electrical business. It means you dont have to go to one supplier who you have an account with, you can go where you like for the best prices. If your tester is lost or stolen or your van breaks down, the money is there, as you cannot work without your tools. The money must then be replenished. Suppliers aren't stupid they love to give you credit, therefore retaining you as you always owe them money. When business is good, i like to buy a bit extra stock like cable or ccu especially when suppliers have them on offer, materials that you use on a regular basis and if you do have a quiet few weeks, you have a bit stock if a job comes in and your not driving to the wholesalers for bit & bobs.
Cheers