It depends on what welding process/electrodes you'll be qualifying. If you're using cellulosic then you'll probably need an aggressive preheat maintenance as well.
R 460.20307 Welding preheating.
In addition to the requirements contained in 49 C.F.R.ยง192.235, which is adopted by reference in R 460.20606, if preheating is required, then an operator shall monitor the preheat temperature to ensure that the required preheat temperature is reached before beginning,
and is maintained during the welding operation.Common preheat temps using EXX10's with heavy wall (.844) are typically north of 200F to maximize hydrogen diffusion but also minimize stress in the weld joints. How will you apply the heat, how will you maintain the minimum through the welding process are questions you would need to answer in the procedure. Holding 250F isn't as simple as it might sound without some pre-planning. If using low hydrogen electrodes/processes then this number can come down significantly - I've seen it specified when temps are below 50F to heat up to 70-150F, and at all times between 70-150F. In any case, since procedure qualification is required per API 1104, the minimum temps you establish are essential variables.
In this study commissioned by federal DOT, preheat was established at 250F.
https://ntl.bts.gov/lib/44000/44200/44214/44214.pdf