What is the position of the parent who connects you to that Thai ancestor?
Thailand has no grandparent rule. Nationality passes from parent to child one generation at a time, with no limit on how far the chain runs - so a Thai grandparent or great-grandparent reaches you only if the parent in between is Thai. That parent's position is the whole question. Bear in mind that Thai nationality by birth is automatic, so someone can be Thai without ever having held a Thai document.