Electrical Main Panel

Here's a thought, and @CudaFish1968 could tell you if it will work and is NEC legal. Remove one double pole breaker from your existing 200 amp panel, and add your 100 amp breaker to feed the new sub panel. Move the circuit that was fed from the breaker you removed to the new 100 amp panel and install the correct size breaker to feed that circuit. Unless there is something wrong with your existing 200 amp panel, it would save you several hundred bucks and a crap ton of trouble to do it that way.