How Cuban am I?

Your ancestry is
1/2 of each parent
1/4 of each grandparent
1/8 of each great grandparent.

So if your great grandfather was totally Cuban then your ancestry is 1/8 Cuban.

DNA wise it is not that simple. You really don't have your ancestors' blood. The terms "blood related" and "bloodlines" are still tossed around in genealogy today but they are not scientific terms. Your blood contains your DNA but so does your skin, your hair, your bones, your saliva and pretty much all your body tissues and fluids, So it would be just as correct to say "bone lines" or "skin related" as bloodlines or blood related. If you got a blood transfusion from a Irishman it would not make you part Irish.

Your DNA contains your genes. 44 of your 46 chromosomes are Autosomal and you got it 50-50 from both parents but not normally 25% from each of your 4 grandparents. The reason why because of "meiosis" there usually will be some bias in what you inherited from grandmother and grandfather on both sides of the family and how you inherited the bias will not be how your siblings inherited it unless you have an identical twin. Understand "meiosis" has been going on in your ancestry since the beginning. Everybody just gets different chunks from different ancestors.

You are not Cuban at all, you have some Cuban ancestry, Your ancestors don't 'make you' what they once were

You aren't Cuban at all unless you hold Cuban citizenship. You have a fraction of Cuban ancestry. If your parents otherwise have Mexican ancestry, but you were born in the U.S., then you are an American citizen with Mexican and Cuban ancestry.

You are person with Mexican and Cuban ancestry.

Hmm! Just a small %