Views,
I can't say 100%, having never worked with recipes this way, but it is most certainly percent
by weight.
Judging from a few bread recipes in the King Arthur Flour Baking Companion, about 12-15oz of flour makes a 9x5" loaf. Since 15oz gives you 425gm, the easiest conversion for you would be 10gm for each percent of ingredients. You could go back to ounces by dividng by 28.35, but if you've got a digital scale, it should do grams.
Round to convenient amounts --- which will probably but not definitely work:
Rice Flour: 15 oz (by weight -- don't try to use volume for flour, it varies a lot)
Water: 1 1/2 cups plus 3 tbs
Sugar: 3Tbs (sugar and starches are less dense than water)
Tapioca Starch: 3 Tbs (this is violating the above rule, you're only talking small quantities here)
Corn Starch: 3 Tbs
Soybean Oil: 2 Tbs
Methocel K4M: 14 gm -- should be 2 Tbs (half the density of water)
Salt: 1 tsp (salt has a little more than twice the density of water)
Yeast: 2 tsp
Resource:
Bulk Density Chart