That's when things break. Alternatively look at the size of blocks you are using. Check the manufacturer's data to see what the breaking load is. Assuming the existing blocks don't break, it would be wise to choose a rope with a similar breaking load. You will probably be surprised just how small you can go.
Smaller sizes bring lots of benefits.
A word from the riggers Ben Wood specialises in big boat rigging at Hood Yacht systems in Burnham on Crouch and is currently preparing all the rigging for the fast growing fleet of Mumm 30s. Since last year he has successfully been using a hybrid material called TD5000. Made from a mixture of Technora and Dyneema, the material is claimed to offer the best properties from both fibres. For 1997 Ben tells us they have a new rope called TD6000 using a finer mixture of the two fibres. Spliced, stripped and handled in the same way as Spectra and Vectran, TD6000 is claimed to offer better properties than either of its constituent fibres at a price only slightly greater than that of Dyneema alone.