Here are the numbers that I use to calculate the amoutn of priming Sugar that I use (based on actual Sugar, not syrup, but that should be easy to convert):
4 grams of Sugar (Sucrose) per liter will produce 1 volume of CO2.
This might need some more explination. You will have notived that not every beer has the same carbonation and you can easily vary this yourself when home brewing. Some guidelines for different styles:
Ales: 1.5 - 2.0 Volumes of CO2
Lagers: 2.0 - 2.5 Volumes of CO2
Stouts: 2.0 - 2.5 Volumes of CO2
So the amount of priming Sugar will depend on the beer that you are producing and the amount of carbonation that you would like to add. It is important to note that fermented beer already holds around 1 volume of CO2, so you will have to subtract that in your calculation.
To prime a 330 ml bottle of Porter (which is what I am doing right now), looking for 2.5 volumes of CO2 (I want it pretty foamy):
2.5 - 1.0 (the residual CO2) = 1.5 Volumes of CO2 to add
4.0 (grams of Sugar per volume of CO2) x 1.5 Volumes of CO2 to add = 6.0 grams of sugar / liter of beer to add 1.5 volumes of CO2
So for bulk priming 20 liters, one should add 20 x 6.0 = 120 grams of Sugar
and for 330 ml bottle priming, one should add 0.33 x 6 = 2 grams / bottle
It is much easier and much more accurate to add 120 grams in one go to the whole batch of beer, over adding 2 grams to each bottle. You might also have bottles of different sizes (I always do), in which case bulk priming has another benefit.