Tire clearance isn't what you should be worrying about but more matching you tire to the rim. If you use a super wide tire, you'll be losing contact area because it pinches it up and changes the tire's profile thus reducing your footprint. For most applications, this is not a good thing. I've found that the 140 works just dandy for the rear KLR rim. Note that I also run low pressures on the street. If I were going to run the high pressures that a lot of folks use, I'd prefer a 130 in the back. If you are riding in deep sand with very low tire pressures, a 150 or 160 If you can make it fit, will do quite well but you'll have real problems on the street.
Percentages mean nothing (90/10, 50/50, etc.) If you ride 99% of the time on pavement and the 1% you ride offroad is in deep sloshy mud, a 90 street/ 10 offroad simply wont cut it. You need a tire that will handle the worst terrain you encounter.
If it's a very light covering of sand on hardpack, a more aggressive tread pattern will be your best bet. In deep sand the idea is to float on top but sand on hardpack is like grease. You need to cut through the grease to get to something solid. A wider tire will make you float more and reduce your traction. Sand is a bit like snow. If it's deep, traction is good but if it's just a light coat on top of something hard, it's pretty darn slippery. That is about where the similarity ends btw. For what you've described, I don't think a wider footprint will be much use. Contrary to what you may have been lead to believe, wider isn't always better.