How to make guitar sheet music reflect string change?

I'm trying to create an instrument in the .xml file that is a guitar tuned down to C, so that the strings are C F A# D# G C, but the sheet music is written as if the instrument is still in E. For example: In the sheet music you put an F, in the tab it should be a 1, but it should sound like a C#. I'm stuck either with a method that doesn't accurately transpose (transposing down by 4 steps resulted in certain discrepancies like the key of F major being written as the key of A major, rather than Db major). While I could technically just keep it in C and write the whole thing how I want, then throw it up 4 steps, it takes a lot more work overall and I want to make this a quick and easy process for myself so that I can write it once through without having to do any math in my head of how it should look. I've also tried transposing up 16 steps, but this resulted in the instrument being an octave higher than I wanted.