Also, you might want to check out an article in the technical upgrades section written by one of the members a number of years back:  "Spartite - 12 Hints for Installing" (I can't seem to get the link to work correctly, for some reason): ​www.catalina36.org/members/technical/upgrades/spartite-12-hints-installing

On our boat, it's not so much that the partner is not centered as much as a slack rig results in the aft end of the mast actually resting on the aft end of the partner.  With a properly statically tensioned rig (including pre-bend and rake) our mast comes off the partner and becomes more centered (although not exact) and ideally this is the point where wedging should done.  Like ​Tom Sokoloski mentions, this is where he substituted wedges by block and tackle prior to pouring in the Spartite.

Match man - the link is bad what was the product called so we can search on it?

Three major reasons for wedging:

• Without the wedges, the distance between the spreaders and the mast step is such that adding additional rigidity along the span is desirable.  Adding rigidity to the length of mast between the base and the partners help ensure that the mast butt sits squarely in the mast base and therefore does not put localized strain on any edge of the mast butt.
• In almost all most boats, the amount of mast flex at the partners during high rigging loads will cause the mast to bear on the partners, causing excessive localized stress here too.  Usually the forward edge of the mast will bear on the forward edge of the partners due to the mast driving the boat forward during high mainsail loads.  In some cases, the hole in the deck and the position of the step are such that one side of the mast will rest on the partners.  Wedges tend to spread out this contact area.
• Finally, wedges immobilize the mast and make sealing the gap (theoretically) easier.

Ben:  Just taking a wild stab and thinking that the uneven pressure on the mast butt thing is where you don't have buy in.  Here goes.....

Visualize a 10' solid round pipe 1" in diameter sitting exactly vertical on concrete floor with 100 pounds on the very top of it.  The 100 pounds would be distributed over about 3/4 square inches of base.  Now, tilt the pipe 45 degrees.  The 100 pounds have not changed, but the contact area with the floor is no longer 3/4 square inches, so the 100 pound load becomes more concentrated in the remaining contact area.  In engineering terms, this is called point loading and there are two primary ways this is dealt with:

Now, I know our masts don't tilt 45 degrees and are not rigid like a pipe.  This was used to help visualize tilting of a base and obvious point loading.  Point loading starts at anything past vertical and if not dealt with be transmitted to the mast heel, solid or hollow column.  The amount of downward force on a mast base is roughly equal to the displacement of the boat, so point loading (if left unchecked) on a Catalina 36 can become 7 tons on just a portion of the wall thickness of the mast.  Personally, I'd like that not to happen too often....

All of the downward force certainly makes it to the heel and all of the tilt tries to makes it to the heel too, but is counteracted along the way.  This is point loading solution #1.  The more correction you have along the way (like lowers and partners) the less point loading you'll get at the heel because you're adding rigidity, thus decreasing tilt, and thus decreasing point loading of the mast heel.  This is the way that Catalina and US Spars (Charleston) have engineered our standing rigging.

Finally, solution #2 to point loading is to spread loads over a wide area.  Selden certainly recognizes that point loading of the mast heel is to be avoided as they provide a convex attachment to their mast heel as a mitigation technique.  Page 22, lower left hand corner, here:  ​http://www.riggingandsails.com/pdf/selden-tuning.pdf.

The backstay, forestay and two cap shrouds are all 5/16" (8mm) and the breaking strength of each wire is 12,360 pounds.  If you tighten all four to 20% of breaking strength (which is the upper end of the first static adjustment recommendations) then 4 X 2,472 pounds = 10,000 pounds of downward pull.  You're at 70% of displacement already and all you've done is an initial tightening of 4 wires.  Add in the downward pull of the forward lowers that you tightened to induce pre-bend, add in the weight of the mast, boom, rigging.  Go tune for performance and you'll most certainly increase the backstay tension to remove forestay sag, perhaps creeping this up to 25% of breaking strength which increases the headstay tension to about 30% of breaking strength.  Viola, you're likely close to boat displacement and you have not even added in the dynamic loads imposed by the halyards, sheets,and the not-too-insignificant load of the air on the sails.