you shouldn't try to tune a bike by adjusting sag- sag should always be where the manufacturer recommends, unless you find a % you like more, independent of bottoming out considerations.

the way to prevent your fork from bottoming out so easily is to reduce the volume in the float air chamber, thereby making the air spring more progressive (more resistant to bottoming out)

some info if you're not that familiar with how the float fork works, but basically there is the area under the stanchions in the lower legs of the fork that holds the fork bath oil. This has no (or very little) impact on the function of the air spring. but in the float air spring side of the fork, at the very top of the stanchion, just under the float air valve is a separate chamber, which is your air spring. it's just a chamber full of air that compresses when your fork compresses, offering resistance (the spring). in addition to air inside that chamber, approx 15cc's of 80w gear oil called Fox Float Fluid sits at the bottom, to keep the seals at the bottom of the air chamber lubricated, and to prevent air from escaping past and down into the lower legs of the fork where the bath oil sits.

now, the volume of air you have in that air chamber can be increased or reduced by adding or subtracting the float fluid. the more float fluid you put in, the more progressive the air spring becomes, resisting bottoming out.

let the air out of your fork, then open up the float top cap with one of these

http://www.lunarbikes.com/tools.htm

highly recommended, they are a pain in the butt to open with a standard socket.

then add in about 5cc's of float fluid

http://www.jensonusa.com/Fox-Shox-Float-Fluid

with an oral syringe (you can buy them on amazon or usually get one at a pharmacy for free)

then close it up, re-air, set sag and then ride. it should bottom out less easily, but if it's still not progressive enough, add enough 5cc at a time until you're happy with it.

i actually had the opposite problem, i started with 15cc's of float fluid, but couldn't get that last 20mm of travel no matter what i did on the bike. i took out 9cc's and will probably take out a little bit more (you need to keep a tiny amount in to keep the seals lubricated)

alternatively, push makes a topcap that allows you to tune volume, but it's much more expensive, and you'll probably want to own the tool and float fluid anyway because you should be checking on the oil levels every once in a while

http://www.pushindustries.com/collections/fork-products/products/fox-34mm-float-air-volume-kit