It depends on the type of weld your trying to do but for a drop the tongs weld I always use 2-3 welding heats. All I do with the 1st heat is get the right temp & tap them together so they have stuck. then I reflux & put it straight back in the fire. It won't take long to get back to a welding heat & this time I work it with medium blows. A 3rd welding heat can be used to "dress" the weld up. This method works well as it makes sure you have a full heat through the steel on the 2nd weld, removes some of the guess work.
Borax as it comes has always worked well for me.
Fire welding is a lot about getting a "feel" for it, if you get it right you can feel it stick. So good luck & practise, practise, practise.:)