Does stacking a 40% Blacksmith elixir (bought) with a 23% Blacksmith potion (made) = 63% improvement or does the last potion I drink mean the gain?

I drank both, but it's hard telling if the results were applied since the best I could get was Exquisite.

Thanks.

No. I believe no potions stack at all (except those health, stamina, magicka potions).

Potions don't stack, only the last is used. Restore potions is instant, not sure if buff food stacks.

Fortyfy skill enchants stack and stack with potion. Same for resistance potions and enchants so it makes a lot of sense combining them.

A resist 40% fire enchant and a 40% potion is both pretty easy to find and reduce damage of dragon fire to 1/5.

That's what I thought. Thanks for confirming.