I cook potatoes by boiling one third as much water as potatoes.

mi jukpa lo patlu lo nu vei mo'e fi'u ci loi patfu lo djacu cu febvi

Suppose I have 300 grams of potatoes. fi'uci loi patfu is 100 grams of potatoes. "mo'e" converts the following sumti to a number. "vei" is needed when applying a complex number to a sumti as a quantifier (the terminator "ve'o" is not needed). So vei mo'e fi'u ci loi patfu lo djacu is 100 grams of water ("100 grams of potatoes of water" doesn't make sense).

To say explicitly that it's 1/3 by mass, say:

mi jukpa lo patlu lo nu vei mo'e lo niltilju fi'ucisi'e loi patfu lo djacu cu febvi