The original CAL 21 mast would be 26'0" extrusion length (same as the CAL 20) so if your mast matches (or nearly?) matches that length is is original or at least is the correct length.
I figured you would cut off from the top due to that requiring less modification of rigging. If you cut off from the bottom you will need to move everything up or else shorten all the standing rigging (instead of just the backstay and jumpers). I still don't think that shortening the mast will really make enough difference to notice, especially considering the extensive work to make the mod. You will only be reducing the height of the center of sail area by that 12-14", with a triangle shaped sail, most of the heeling force from that sail is towards the lower half of the sail (given stock CAL 21 sail plan the center of the sail area is 8'6" above the cabin top) reducing that to 7'6" or even 7'4" will make virtually no difference in the heeling force of the sail plan, The slight reduction in mast weight will have a negligible effect, if any.
Now, if it did turn out that your boat DOES in fact have a "non-standard" mast, that is longer than 26' and/or a larger section extrusion........ reducing the mast down to the original dimension of 26' might be worthwhile....... then again... it still may not make enough of a difference {although, it would not be a bad idea to get the spar back to as close to the original (Lapworth design/Jensen built) specs.... still best to consult with a spar maker or yacht designer, (or a friendly mechanical engineer?)}
I have a copy of the original CAL 21 Sail Plan as drawn by Bill Lapworth, however it is too big to scan. The brochure pics below are a good start.
So, anyway...... your boat, your choice, but I still think that shortening the mast is a lot of complicated work for very little if any gain... and possible will make thing worse in the long run. It would certainly reduce the resale value.
How old are your sails?? Believe it or not, older sails that have stretched out will increase heeling effect and reduce the driving power of the sails. We replaced our sails after 23 seasons and were "blown away" (pun intended?) by the improvement made by new sails! Adding a roller-furling jib also made sailing much easier for my Dad as he could then easily and quickly reduce sail by furling the jib, then increase hte sail as hte wind died back down by unfurling hte jib. I suspect replacing or at least "retuning" the sails wil lhave more effect on "taming" the boat and cost considerably less than modifying the mast height. A local Sailmaker can inspect your sails and recommend repairs/updates or else yo ucould send the sails to a company known as "SAILCARE" that will make any needed repairs, then recoat the sails with a resin to restore their life. As long as your sails are not terribly stretched out, this will be a very viable idea. Again, retuning the sails like that will help to reduce heeling while improving light air (and normal condition winds) performance. I'm not saying it will "turbocharge" the boat, that seems counter to what you wish to acheive,.... but that better sail shape goes a long way towards easier sailing and more enjoyment, improving control of the boat. It might even be worth having hte mainsail recut to slightly reduce it's area by eliminating the "roach" or outward curve of hte trailing edge, you won't really miss that area, but it will make handling hte sails easier (and might breath new life into a tired sail as well, for less $$ than new or rebuilt sails). Plus, a new mainsail if you were to sell the boat is going to be less $$ than a new mast of the original height.