I think Maple Syrup is about 31 ppg, so if you were looking for 3 points of gravity in 5 gallons you would need

31 ppg / 5 gallons = 6.2 points / pound; then 3/6.2 * 16oz / pound = 7.7oz (by weight, about a cup).

Likewise for the brown sugar, I would imagine that is about 40ppg. So if you wanted to get 3 gravity points total for the carbonation, it would be

40 ppg / 5 gallons = 8 points/pound; then 3/8 * 16oz/pound = 6oz

So, if you wanted to do half the sugar from syrup and half from brown sugar, you would use 3.4oz syrup and 3oz of brown sugar. For the brown sugar, I would dissolve it in a bit of hot/boiling water before adding it to the keg.

You might want look up the amount of gravity points you need probably using an online calculators to tell you how much, if it tells you in corn sugar or table sugar then you can convert that back to gravity points in 5 gallons. Once you know that, you can calculate the amount you want.

Another way to look at it (and simpler) is just as ratios from what you get from an online calculator. If it says 4oz of corn sugar, then you could just adjust as compared to what you are using. So, for syrup:

4oz * ( 46ppg / 31ppg ) = 5.93oz

I hope I didn't make that seem too complicated, because it isn't.

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