How much biological washing powder should I use to clean my bath?

[WalkinginaWonkaWonderland]

Have the biological powder and the scummy bath at the ready but this is my first attempt and wondered exactly how much powder I'm supposed to use? Want to be gleaming for Christmas

Have you got an ecloth? One of those will do a fantastic job without any chemicals at all

(AKA microfibre cloths, you can get them at any supermarket)