Thats the problem I don't either, I just wing it.
That drop off is proving it to me.
Started out ok, thought it was going well, on the 3rd set of spots things started going downhill, kept either burning through or grinding thru, ended up welding way too much to try & build up the back so I could get the front nicer. got it too hot & warped it pretty good, hammered most of that out , ready to sledge-a-matic the sucker at this point.........................