I too flit about between liquid , liquid tabs , powder and powder tabs - whatever is on offer at that time and whatever takes my fancy lol!!
Im in a hard water area and for a nappy wash ( which im assuming is what we're talking about? ) i use:
liquid tab = 1
Powder tab = 1/2 , or 1 if really dirty (pooey

)
Liquid = i always use the double concentrated so use half a cap full , 15ml , which works out to 30mls if normal concentration iygwim?
powder = i use this the least but would use a couple of tablespoons i guess?
Ive never had any problems with anything.
xx