Rule of thumb says from the length between hounds and top not more than one third.
Actually that is not the rule of thumb at all, but an overly conservative rule initiate by Bill Roberts and followed by Bimare when they fit hooters rather then spinnakers.
All modern spinnaker boats have the spi hound at about 60% hounds-top distance. All alu masts should be able to withstand that when the spi area isn't significantly larger then 125% the mainsail area. With carbon masts it dependent on the lay-up and local reinforcements.
The dependence between flexing and spi hound higher is a very sensitive one (3rd power relationship) and going to 30% will see the flexing reduce by a factor of 8 where the stresses inside the mast will be lowered by a factor of 2. If you are unsure about your safety margin then go to ratio's of 50% or 40% but not 30%, the latter is really too small.
An example. The F16 use the unmodified lightweight aluminium mast of the Taipan 4.9 who developped their mast profile from the Alu masted A-cats of the 80's. The hounds on the Taipan are at 6000 mm from the mast based and the top is at 8500. We fitted all our spi hounds at 7500 mm or indeed (7500-6000)/(8500-6000) = 0.60 = 60%. The only mast that was broken under spinnaker sailing was in China where the owner fitted a F18 spinnaker with a higher mast hound then that and the mainsheet accidentially uncleated while they were 2-up beam reaching from the trapeze. The F18 hound fitting is about 8250 mm from the mast base or 750 mm (2.5 feet) higher then 60% rule.
Currently the new F16's are using higher stay hound hieghts of about 6100-6150 mm but this is unrelated to spinnakers. They do so to limit the (former) increased flexing of the mast top under the (much increased) square-top sails (40%) that the new F16's have over the old Taipans (20%). But even with this modifications the spi hound is still at (7500-6150)/(8500-6150)= 0.57 = 57%
The F16's founders chose the 60% rule after finding that the F18's, F20's and other modern purpose build spinnaker cats used the same rule. I think the only exceptions where the earlier F18HT designs (Bimare again). I've used this rule also on a Prindle 18 and never encountered problems there.