Hi Marc
I don't think that this is possible on the standard system. While there is still an uncovered requirement, MRP will generate a new replenishment proposal.
As already mentioned here, you can use a BAdI to reduce or round down the requirement quantity. The correct BAdI to do that is MD_CHANGE_MRP_DATA.
See this blog with an example of this BAdI:
Making an element not relevant to MRP or MD04 using BAdI MD_CHANGE_MRP_DATA
BR
Caetano