I would advise you to look at Robert Ganier's post in the thread the Dog linked.
I, of course, spent great amounts of time this spring measuring and adjusting to equalize my spreader angles. I erected my mast, as only I can handle an erection, and the damn things were still basically horizontal-where they've apparently been since about 1973. I have them well seized to the shrouds, so am unconcerned about slippage. Furthermore, for all the Cartesian geometry displayed, I see no allusions to the fact that the spreader does not apply it's strain to the mast proportionately angle-wise. My casual observations show that most seem to be closer to horizontal than bisecting the shroud. Whether correct or not, this seems to be the case. In short, I suspect the actual angle is not critical (how accurate can you get it?) and while upwards to a degree is desirable, slippage is intolerable. JMHO, YMMV
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.