Is there any math/calculation to decide how many avg. days would it take for me to raise my serum ferritin from 25 to 70 on 65mg elemental iron/day?

Basic mistake: Lab values are indicators to tell you what's going on, not something to be treated or target values. I'm much more concerned about finding out why you're iron-deficient -- you're a young man, and if you're not an ill-advised vegetarian, it's ulcer, cancer or hookworm until proved otherwise. Your question is also unanswerable since we don't know how fast you're losing blood. Glad you're proactive.