Make sure you always adjust your cutting height depending on numerous variables.

-  Take into account the average rainfall for the current time of year.  If it has not been raining at all try and cut your St. Augustine around 4" to help keep it resistant to drought.  If you have a properly functioning irrigation system I suggest cutting the grass right at 3".  I personally think floratam and St. Augustine look excellent when they are maintained at 3".  However some lawns are different than others and can never be made to look good.  For example if a lawn has been let to grow too long for an extrended period of time the Floratam becomes extremely spongy and the tractors like to sink down into the grass forcing you to raise the cutting deck.  I think the only solution to the spongy grass is to have it de-thatched or replaced.