lincoln tig 250

Experience/trial and error. You can find some guidelines in the owners manual. I got a video with mine. I've had it for awhile.

If set too high, metal flows too much or blowthrough occurs. If set too low no penetration and weld fails, large lumpy bubblegum looking crappy joint.

As stated above, Trial and error. Speed and feed is another factor.

Are you Tig welding or Stick welding?

If tig welding then your heat will vary in the process by means of a foot pedal or rheostat on the torch. This takes a lot of practice.

If stick welding, it is as above, trial and error, but ask the people where you bought the rod what the maximum amperage is for that type/size rod and go from there based on the size(s) of metals you are joining.