running backstay leads

[Brion Toss]

Hello,
The source of the load on the runners is the forestay. Because some of the loads are lateral, and because the mast and aft lowers contribute to resisting the forward component, runners typically can be sized lighter than the forestay. A simple rule of thumb is to make the runners one wire size lighter than the forestay, and this seems to work pretty well, assuming that the forestay is in fact the correct size for the load, and that the angle of the runners is similar to the angle of the forestay.
Once you get that figured, the block selection should be easy. Note that in sailing gear, rated load is typically about half breaking load, though not always; best to check with the manufacturer.
If you do switch to HM, be sure to take constructional elongation into account.
Fair leads,
Brion Toss