For what its worth, a square yard of 0.5oz kite doesn't weigh 0.5oz (if you don't believe me, get yer scissors and yer kite and conduct an experiment
... but it really doesn't matter.
The 0.5oz or 0.75oz is really just a general indication of what the kite is suitable. For example
0.5 oz = lightweight spinnaker, used in light breezes (0 - 10 knots?)
0.75oz = general purpose spinnaker, used in moderate breezes (5 - 18 knots?)
1.5oz = heavy weather
spinnaker, used in heavy breezes (15 - 30 knots?)
2.25oz = bulletproof spinnaker, used in extreme wind
(25 - ? knots depending on how big your cojones are!)
Of course, the above figures are only approximate - your mileage may vary. Most people cruising, short handed, probably wouldn't carry a spinnaker above 15 to 18 knots anyway.
Just as an aside, if you are flying a lightweight spinnaker in breeze of less than, say 8 knots, it is really worth using lightweight sheets
. I have 3/8" sheets
for most kite work, but use a 1/4" sheet for light breeze. Also, if you run a sheet and a brace (a.k.a. "guy") on each clew, remove the guy from the free clew in light breeze (you can either re-attach it prior to jybing, or just use the sheet as a brace (guy).