I first read @Horton thread on Volume vs Intensity last year. It was always in my head, but seemed like all my buddies that I skied with on the river wanted me to shorten every time. I decided to actually follow my desire to run VOLUME this summer and avoid the peer preasure to run a PB every set.
My suggestion is to run a TON of buoys at line lengths and speeds that are achievable every time with GREAT TECHNIQUE. It does no good to bang your head against the wall trying to run your almost or true hardest passes. In your case, if you can comfortably start at 28 MPH, I would run 2 passes at 28 MPH and then 4 passes at 30 MPH. If you can ski every day, I would do that 2 sets a day for 4 days in a row. On Day 5, run down the speeds to failure and go 28 MPH, 30 MPH, 32 MPH (if you run it go to 34 MPH, if not drop back to 32 MPH) and finish out the set with your last successful rope length (likely 30 MPH?) and run MORE buoys. On Day 6, I would ski 2 sets of 2 passes at 28 MPH, and 4 Passes at 30 MPH. On Day 7, I would NOT SKI AT ALL! REST!
This summer I spent about 90% of my practice time at 32' Off working on finishing my offside turn. I really only ran 35' Off in tournaments (And a few days where I ran it 28, 32, and four 35's). I think I ran 38 in practice about 4 times and tried it about 8 times total. VOLUME netted me running 38' Off in 3 different tournaments this season, way more buoys at 38 than last season, and a new PB of 2@39.5' Off. I am a firm believer in practicing a line length that is achievable with GREAT Technique. Next thing you know, that achievable pass moves up to the next line length or speed.
Stick with VOLUME and it will net higher buoy counts. Use intensity infrequently.