70% is a little high or the other way to look is leaving 30% free for OP (Over-Provision). A SSD is similar to hard drive as more full the drive gets, the write speeds can slow down (has no effect on read). This steams from OP/GC/TRIM needing free space to work with. Here is a thread on the OCZ forum, which goes into more details... http://www.ocztechnologyforum.com/forum/showthread.php?85029-Understanding-SF1200-drives-TRIM-OP-area-use-and-Life-write-throttle
I recommend keeping roughly 10 to 15% free and limit (or avoid) running benchmarks, as they quickly eat up your OP and can cause the drive to throttle (more seen on Sandforce SSS's).