If the consumtion is constant and only the steam production is affected during startup it should actually be rather simple. Just see how long it runs on X buckets of nitro diesel and multiply by the amount of burn value it goes through each second (this should be 40000/48000*1600= 1333 1/3 ticks of fuel per second. I will be back later once ive tested it
Edit:
10 buckets of nitro diesel lasts just about 2½ minute in the boiler or 150/10= 15 s per bucket. This means each bucket should produce 15*40000=600k steam while at full efficiency. Since the energy value is 512k EU in 1 bucket of nitro diesel we could translate this to 1.17 times the eu value in steam for other fuels.
Using a large steam turbine with 140% rotor this would give a net output of 420k eu per bucket of nitro diesel or an efficiency of 82%. Just 3 % below the turbo diesel generators.
Not as bad as i thought but still not very good. You should be able to run a tungstensteel boiler constantly with a distillery setup run with oil (not heavy or light) with some excess as long as you run the process mostly on the byproducts (sulfuric acid, naptha, methane, LPG) and produce 1400 eu/t in your voltage tier of choice with a large tungstensteel boiler and 1-2 large low pressure steam turbine using 140% turbine blades