That question, interestingly, has helped to divide some of the reparations forces here, between those arguing for a 1652 (Dutch East India Company settler landing) starting point (one could go earlier, to Portuguese mercantilism), versus 1948. I think that debate takes us nowhere at this stage. But it has unfortunately led to important splits on the left about where to move the reparations agenda.
The key thing, I've been arguing (maybe or maybe not correctly) is to locate specific incidents and institutions that need disciplining, like the World Bank's $200 mn in loans to apartheid SA, including $100 mn for white folks' electricity (while townships were still dark) between 1951-67. That money plus interest should be repaid, not just for past justice, but to enforce the point to int'l lenders that if they fund dictators they may get called on it in future. Once a few key institutions are nailed, it will be easier to get general principles of liability established. The WB and Citi are good targets, but there are plenty others, including apartheid sanctions-buster Marc Rich.
There's a broader movement for joint creditor/debtor liability to justify debt-cancellation calls, and it usually goes from the int'l legal principle of "odious debt," via which the US refused to pay Cuba's foreign debt to Spain at the turn of the last century, after a change in colonial control. The Bolsheviks used the principle as well during the late teens, so there's something there...