You can use the WRC diagram or better yet, the modified WRC diagram by Kotecki and Lippold, to estimate the ferrite number of the diluted weld.
I would be suspect of using 304 filler metal when welding to the carbon steel. There is a good chance your ferrite number would be too low and cracking can be expected due to LMPC.
Best regards - Al
Back in my days of erector engineer I had to weld the refractory anchors of three crude oil furnce heaters. The heaters' shell was carbon steel and the anchors were made of high alloy steel. Many years have passed since then and I don't remember what kind of alloy steel was it.
The anchors were welded with stick electrodes of a composition compatible with that of the anchors, and were specified by the heaters manufactrurer. Again, I don't remember the electrodes type. Neither the WRC 1992 nor the De Long diagrams existed in those days.
Clearly, and Al agrees with it, a 304 type electrode is insufficient for that application. Remember that no chain is stronger than its weakest link, and it's useless to have an anchor that withstands the sun temperature welded with a material that will melt with boiling water. (I'm exaggerating, of course).
The cost of the welding consumables is little compared to the cost of the finished vessel, i.e., ready to be put in operation, so, in my opinion, welding consumbles is not the right place to save money.
Giovanni S. Crisi
Sao Paulo - Brazil