Probably, but I would do it a different way.
You can work out a pppg ((gravity) points per pound per gallon) by doing some simple measurements:
- 1 gallon = 128 fl oz
- 1 pound = 16 oz (weight/mass)
So, you could test 4oz (weight/mass) of your syrup/honey in 32 fl oz of water (or 1oz in 8 fl oz. water, or 0.25 oz in 2 fl oz water, etc.), using a hydrometer or refractometer like you normally do for wort. You should be able to measure the cider directly, without needing to mix it with the water, since it is serving as your water.
I usually do this when I'm working with a new syrup or honey that has an unknown specific gravity. Then I plug those numbers into my brewing software and let it do the full-volume calculations.
Another option would be to mix up a sample using the same ratio that your must will be, and measure that:
- 80 cups of cider
- 2 cups honey
- 1 cup syrup
Which would work out to be, in the smallest measure that is normally found in a kitchen in the US:
- 1 cup + 4 teaspoons cider
- 1/2 teaspoon honey
- 1/4 teaspoon maple syrup
Mix that together well, measure it, and drink your sample must.