Harken has loading formulas which are relatively accurate--they are available in the catalog and in their website. In general, the loads on E27s are fairly light, so unless you are using micro-blocks and very light line, you shouldn't be seeing failures with newer hardware.

For lines, workable size now the more determinate factor--it's awfully hard on the hands if your spin halyard is 1/4". If you use spectra or vectran line, then you will get plenty of strength for all purposes. Here's what I use:

:: I've had a couple of failures recently and am replacing
:: old lines and changing some other hardware. I read the
:: articles about tuning and recommended line sizes but
:: when it comes time to buy cleats, block, and lines, I
:: often don't know what to get. Is a breaking strength of
:: 2000lbs enough for a spinnaker halyard? How much load
:: should a mainsheet block or a boom vang block be able to
:: handle? Any guidelines on how to estimate the various
:: loads on the boat? Primarily the halyards, both the
:: lines and the clutches or cleats that hold them, the
:: vang block on the boom, the main sheet blocks, the
:: backstay fittings, the forestay fittings.
::
:: Thanks