Basically ice-cream is a frozen foam of a water-fat emulsion. The desired texture has small ice crystals, so sugar and sometimes alcohol are included for their effect in inhibiting the growth of large ice crystals.
Typically you want about 15% sugar, 15% fat, and 70% water. If using alcohol, it replaces some of the water, up to about 5%.
These proportions are quite flexible, so you can do the calculations without really taking into account density. E.g. if you're making 500ml of ice-cream, 300ml of a fruit juice which is 10% sugar will give a bit under half of the sugar required; add about 45g. And that leaves 200ml cream, so you want the cream to be 35%-40% fat.
If the only cream available is 20% fat (single cream), you either want to provide additional fat from another ingredient (e.g. coconut oil, if that works with the other flavours) or use a lot of cream (bearing in mind that the part which isn't fat is mainly water) and much stronger flavours to compensate for their reduced quantity. E.g. for 500ml of ice-cream you might use 375ml of 20% cream, 75ml fruit juice, 50ml of 40% abv fruit liqueur, and 50g sugar.
Solid inclusions (chopped nuts, crushed biscuits, pomegranate seeds, etc) don't count towards the above calculations. NB it's best to add them, already frozen, part-way through the churning process.
If you can't get in the ballpark of the above proportions, the more radical approach is to look at using the hydrocolloids so beloved of modernist cooking. A starting point can be found in some of the recipes of the hydrocolloid recipe collection edited by Martin Lersch (aka texture.pdf).