The voltage needs to be around 3V. Simple to use 2 dry cells. how do you feel converting to .bas can help you,while , already you have the .bas file with you?
the Nokia3310 displays are piece sensitive. Thus the display contrast changes from piece to piece.
it has a range . Now that you know the contrast value location, in HEX, better try with some sample values 10units step
this can be done by editing the hex under winpic800, and save as a renamed file, every time. one you know the workable range, you can fine tune. All the process takes time
and I fear , there is no shortcut.