How much coal should I put in my Hobbyist Steam Engine so that the (full) tank of water and the coal run out at the same time?
In FTB Infinity 1.2.1 (Railcraft 22.214.171.124) I ran two experiments running a Hobbyist Steam Engine out of a full tank of water:
There are many things which can affect what you are talking about. The exact release of a mod, other mods as well as modpack configs. Many packs balance power generation so multiple ways of generating power are balanced against each other.
Your best bet is to load up a creative test world and create the conditions you are looking for. That way you can control your own variables like whether or not the engine is cold, whether you will consume 100% of the power generated once the engine's buffer fills up, etc. This is what I did to get the results above.