Composite masts can be more difficult to determine how much laminate is needed.  In just the last couple of months recall the Gunboat mast giving way and the Sarabi mast mystery.  On an alloy mast you can be quite sure of both the modulus of the alloy and the section properties.  On your self-built composite mast both the actual section properties and the actual performance of your layups might not be easily quantifiable.  What to do?  Borrow an alloy mast.

Actually, borrow a successful alloy mast from the same size/weight of multi.  There are probably quite a few around, including your own earlier mast maybe.  Remove the diamonds and set it up on blocks at each end.  Put a certain weight exactly at the midpoint between the blocks.  Measure the deflection and record it.

When you have your composite mast, set it up the same way exactly.  If the new one is longer, let it run over the ends, just so the blocks are the same distance apart.  It will basically be a pin connection in both cases, so is close enough.  Add the exact same weight and measure the deflection.  If your deflection is greater, you can add more carbon until it matches or even exceeds the alloy data.  This way you don’t even need to know your exact carbon modulus.  I use 25 kg only as an example.  Use enough weight to give visible deflection.