The late boil (lateboil:) feature is for spices and fruits added at the end of the boil. It is a relatively simple dosage scheme of either ounces/gallon or grams/liter. 

The challenge is converting recipes to this dosage ratio. 

Recipes typically have the entire amount of spice or fruit needed for the entire batch. Examples are 10 pounds of blueberries or 4 grams of cinnamon. 

Blueberry example: 10 lb / 5 gallon
= 2 lb / gallon
= 32 oz / gallon
Convert to metric: 
=  32 * 28.3495 = 907.184 grams / 1 gallon * 3.785 = 3.785 liters
= 907 / 3.785
= 239.6 g / liter

This works out to about 10 lb of blueberries for 5 gallons (19 liters). 

Cinnamon example from Autumn Spice beer and Mosher (2004; p. 169, Gingerbread ale): 4 g in 5 gallons
= 4 g in 19 liters
Convert to US: 
= 4 * .035274 = .141 ounces in 5 gallons
= .141 / 5 = .028 ounces / gallon

Key ideas: 
1. Get the weights and volumes in the same unit system, if needed. 
2. Convert both weight and volume to the other system. 
3. Divide the weight by the total volume to get the weight needed per unit of volume. 

##Update
A potentially easier method is to express the weights and volumes in both US and metric units first. Next, use weight/volume division to get the ratios for both US and metric. 

