Thanks for that. Yes I could use the scenario editor but as far as writing a module or reverse engineering is beyond me.
I opened up the .dll with hex editor but info made absolutely no sense to me.
Well, I didn't know about your level of expertise. Without the source code of the module you can't change it without reverse-engineering, and I assume that you don't have it.
In this case however, the thing is not that hard to do, especially if you already know what a hex-editor is and how you can change bytes with it. In principle you are searching for a single 8-byte combination and have to exchange it to another one. Chances are that you will find it only once, but it is possible that there are multiple occurrences. You can simply use trial-and-error to find the right one, always only changing one occurrence.
First, go here to convert the number into hexadecimal representation:
http://www.binaryconvert.com/result_double.html
There you enter the number 150000, which will give you the hex-number 0x41024F8000000000. Now you have to reverse the byte order, because such constants are saved in little-endian in the code. It will give the byte sequence 00 00 00 00 80 4F 02 41 . Search for that in the hex editor. Once found, overwrite it with the byte sequence for your number (350000000=0x41B4DC9380000000= 00 00 00 80 93 dc b4 41). Save it (remember to make a copy of the original DLL) and try.
Now this will of course only work in standard coded vessel modules. If there is anything fancy going on in there, chances are you won't get far with this method. But most fuel mass settings are just using one constant, and that's where this should work.