Thanks for the input. I haven't tried the scale method. I have used the metered pumps before.
I've also used the old-fashioned method of measuring one ounce of water into the disposable mixing cup (a 16 ounce plastic Dixie cup), making a tick mark on the outside of the cup, adding another 4 ounces of water, making another tick mark on the outside of the cup, and using that as a template for future mixes. Did the same with 2 ounces + 8 ounces, etc.
Worked well for the fiberglass skin of a rather large pyramid-shaped project a few years ago- McDonnell Douglas "Delta Clipper".