google.com is the most convenient conversion tool, nutritiondata.com is the 2nd most convenient (it is on the search page)
In the google search area just put your question as an equation:
1.5 cups = ? ml
and search and the answer will come out
similarly
3 tsp = ? ml
most protein scoops are 30 cc
The other thing you can do for whole foods is look at the
USDA database and look at the volume measure for a food and note the gram wt and then convert accordingly.
For example, it shows 1 C oatmeal is 81g. There are 48 tsp in 1 cup so 4 tsp is 1/12 of a cup. 1/12 of 81g = 6.75g oatmeal for 3 tsp.
FYI - I can't see that recipe properly measured making 1L - it should come out about about 0.5L