The melting temperature of DNA itself (which is calculated by the number of H-bonds the bases make in any given sequence) is a direction towards the energy delta that needs to acheived in order to initiate transcription. However this calculation is a pure in vitro calculation. Several websites online will add salt conditions and other physiologically relevant scenarios to proximate the in vivo condition, but none will be exactly accurate/identical. So i would suggest you can use the melting temperature of DNA as a good correlate for deltaG needed for transcription.