If you're getting all the way to syrup i.e. using a hydrometer you can calculate your sugar content using the amount of sap and the syrup you make. I'm going to assume you are emptying the pan after the 2.7 gallons. So you divide the gallons of syrup into the gallons of sap. In your case 100 divided by 2.7 equals 37.03 gallons of sap per gallon of syrup. You then divide the 37.03 into a constant 87 which gives you 2.35%. This is a good estimate of your sugar content. You can refine the number by adding in syrup still in the pan and some filtering losses etc. Hope this helps.
Leader 1/2 pint - Kawasaki Mule - Smoky Lake Filter Bottler
24 GPH RO, 2 1/2 x 40 NF3 (NF270), 140 GPH (Brass with no relief valve ) ProCon pump
2013 - 44 taps - 16 gallons syrup, 2014 - 109 taps - 26 gallons syrup
2015 - 71 taps - 13.5 gallons syrup, 2016 - 125 taps - 24.25 gallons syrup
2017 - 129 taps - 17.5 gallons syrup, 2018 - 128 taps- 18 gallons syrup
2019 -130 taps - 18.5 gallons syrup, 2020 ~125 taps-19.75 gallons syrup