Vibraplane: Should it be loaded close to limit?

Dertonarm is exactly right. Same advice applies to the Minus K platforms - you need to have the total load weight (table + additional slab) come as close as possible to the maximum rating for that platform in order for it to work best. This is necessary to fully 'load' the springs.

Hmm, I thought that you could adjust the air pressure to the load?

4est is correct ... simply place your 30 lbs TT on top of V/P ... pump up bladders and then bleed them individually until the top of the Top Plate on the V/P is 3 and three quarters inches (3.375”) from the top of the shelf that the V/P is sitting on ... no extra weight needed

If you change a component that weighs more or less ... just pump up V/P and re-adjust to 3.375”

At 3.5” my system sounds fat and at 4” it sounds lean and lifeless

Gently bleeding bladder to level V/P will bring new meaning to the word frustration ... you will learn to curse in many new language's you didn’t know you spoke

75% to 85% percent of the max load.

Sure you can work w/o extra weight.
And it still does some good for your sound.
If you're satisfied with 60% performance - than you are fine.
If you want the full benefit from your Vibraplane (read: lowest possible resonance frequency - as this is what the complete vibration isolation issue is all about), you will have to go as close to the maximum payload as possible.
Sorry - not my idea, but physic.
You may ask Audiogoner's Syntax or Solong about their journey with standard Vibraplane and then with added weight till maximum payload was approached.
They will tell you that you just can't argue with physic and how much the sonics - especially in the low and lowest registers improved.
Cheers,
D.