A quick rule of thumb that I have found works for me for aluminium tube masts is as follows.

Take a quarter of the circumference of the mast and multiply by the wall thickness of the tube, multiply by 0.8 of the diameter of the mast and then by 12000 pounds per square inch (approximately half of the ultimate strength of 6063 T6 aluminium alloy, this gives a two to one safety factor) This is a rough estimate of the working strength of the mast in foot pounds.

Multiply the sail area in square feet by the distance from the waterline to the center of effort of the sail, multiply by one (the pressure of a breeze of 20 miles per hour, about 16 knots, this being the usual point to start reefing sail). This gives the pressure on the mast from the sail plan. The two should be approximately equal for design of a mast to suit a sail. For extra heavy boats or long distance cruising boats be a bit more conservative on the strength of the mast by reducing the strength factor to 8000 pounds per square inch.

I hope this is of some help.

David