Just measured the luff of my old (like 2004) Gran Segel spi. Did it twice with a 1 meter long stick, so there is a margin of error there, but not significantly large. Luff was measured to be 897 and 900cm long. I also re-checked my source for distance from block to block, and it's 910cm with 1-2cm slack when fully hoisted.
Hmm, something smells fishy here. I'll try to get a longer measuring tape and measure it again with some tension in the luff.