The 1% figure is a red herring and denial of warranty coverage around it is sticking in my craw.
Here's what Santana says about wheel tension for Sweet-16s:
Excerpt (quoting extensively to avoid misrepresentation):
"...3). Although repeated service or premature spoke breakage are known results of the failure to fully re-tension the wheels after the initial seating, some wheelbuilders and/or their customers wrongly believe that these tandem-specific wheels are "high-maintenance," "fussy" or "weak." Actually, as long as these wheels properly retensioned per Shimano's original instructions (after 1000km or as soon as they make noise), Sweet-16 tandem wheels will generally remain tight. Laboratory fatigue tests that simulate years of demanding use demonstrate that a properly tensioned Sweet-16 rear wheel will outlast traditional 160mm tandem wheels with 40 spokes (and 145mm wheels with 48 spokes).
4). What is the proper spoke tension? When using a Park tensiometer the recommended average tension for all spokes is "26." With Wheelsmith's tool the equivalent reading is "95." Loc-tite is unnecessary. 5). Unfortunately, some wheelbuilders and most customers don't have access to an accurate spoke tensiometer. If there is any question as to adequate tension of a Sweet-16 wheel, anyone with a good ear and a piano (or a small "G" tuning fork like those sold at music stores for $15) can make an accurate assessment by plucking spokes. [emphasis added.] While not every spoke of a true and properly-tensioned wheel will vibrate at the same pitch, a plucked spoke that sounds a note lower than an F-sharp [i.e., lower than 89% of spec. tension, comment and emphasis added] signifies low tension. (Among a trio of black keys, the F-sharp is the one on the left). If the wheel is also untrue, it should be taken to a professional. If, on the other hand, the wheel is perfectly true, a conscientious amateur can carefully tighten all sixteen spokes by one-quarter turn. After rechecking for trueness, if the tension is still low this process can be repeated."
The OP should argue that keeping tension to within 99% of spec. is not necessary to prevent rim cracking (even if it were possible to measure it so accurately.) Although it doesn't actually tell us that 89% is OK, Santana's tech. bulletin reassures owners that it is adequate to check that the plucked note has not dropped as low as F#, -- you have to figure out on your own that this equates to 89% of spec. tension (not 99%!). The OP says that his spoke tension was never allowed to get that low. It is irrelevant that "generally [they] remain tight" means "sometimes they don't": the OP's wheel always was "tight", i.e. not looser than 89%, and still cracked. Ergo, if the facts are as the OP relates them here, the rim failed in normal use with care as per Santana's instructions and Santana should replace it under warranty .