1- Your hair thickness (fine hair has 1/4 the value of coarse hair) so the calculations are different based upon hair thickness
2- Your hair density in the donor area: We are all different with different hair densities. The density reflects the donor bank so if you take out say 3000 grafts, and you only have 3000 to donate, then you have used up your reserve. A good doctor will tell you how much hair you have in your donor bank and as you use it, how much remains.
3- What can you expect in the future as hair loss is progressive. Today, for example, you have some frontal loss and you transplant it with 2000 grafts. In a few years maybe you have more frontal recession behind the area that was transplanted and you also develop some early crown loss. If you transplant these two areas, then maybe in time, the crown loss will become more and wider, what do you do then? Did you leave enough hair in the donor bank to manage future hair loss?
The hair transplant conundrum is not so simple as a graft calculator can easily solve. A good doctor who has experience, will address this for you and give you a realistic assessment of your short term and long term needs. I always tell my patients don’t think a year out, because balding is a lifetime problem.