How much safety margin do you really need when choosing blocks?

I have been accumulating blocks as I found them on eBay, Harken closeouts
and other sources and thought I had a full complement but now I am
wondering. .

For example Rutu is designed to take it first reef at about 20 knots so I
figured sheet loads at 25 knots. At 25 knots the main sheet load will be
about 2,750 lb. That is 156% of the 20 knot load. The sheet is a 2:1
"admiral's cup" layout with a single block on the traveler, a double at the
boom end lead to the mast and back to a pair of 40's at the cockpit.

The single block on the traveler and the pair at the mast are 75mm Airblocks
rated for 5,000 lb SWL. More than I thought I needed but I got them cheap.
My problem is the double on the boom. The way I figure it a 90 degree turn
has a load factor of 141% so 3,877 lb SWL should be enough. I have a Lewmar
80mm double with a 4,000 lb SWL but the Harken calculator says I need a
block with a 7,000 lb SWL which costs $550!

Where do you say enough?

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com