Use the SEARCH function. Search preheat and user G.S.Crisi.

He has a formula (post on here somewhere) that I have lost thru the many changes and migrations of PC's.
It uses the Carbon content to determine preheat range of materials.

I prefer review AWS D1.1 Annex normative I.
The PCm es very importan in  Hydrogen control, because, is possible  risk cracking for hydrogen inducen
Dayanara

Here is a method for estimating Preheat

Calculate the CET

CET = C + (Mn+Mo)/10 + (Cr+Cu)/20 + Ni/40

Calculate the Preheat using CET

Tp = 697*CET + 160 tanh (d/35) + 62*HD^0.35 +(53*CET-32)*Q-330 [Degree C]
where: d is the thickness in mm
           HD is the hydrogen content in ml/100g per ISO 3690 weld deposit method
                typical values HD = 5 ml/100g - SMAW, FCAW, SAW and HD = 3 ml/100g for GMAW
           Q is the actual heat input in KJ/mm Q = eff*Voltage*Amperage/(weld speed)
                typical eff  = 0.8 SMAW, GMAW
                                = 0.7 GTAW
                                = 0.9 FCAW
                                = 1.0 SAW

   tanh() is the Hyperbolic tangent

          
If you want to experiment iterative approach

1. measure the temperature of the steel
2. weld the first pass mesaure the time it takes the weld to go from 800 to 500 degree C adjust the preheat to obtain a t8/5 greater than 8 seconds; allowing the HAZ and weld emough time to transform from martensite.