I want to limit the discussion to split-reads. 

Say I have a split-read with the primary alignment as `chr1:100-200` and the supplementary alignment as `chr1:300-500`.

Is the SV position I report `chr1:200-300` or `chr1-201-299` 

Or something else? I know there is a convention for this, and I wouldn't be asking if I haven't tried googling it. 

Thanks!
The SV position you report depends on the format you report in. I strongly recommend VCF as other formats such as BED and BEDPE are ambiguous for exactly the reason you raise. The encoding of variants into VCF records is covered in the Variant Call Format (VCF) specifications document at https://samtools.github.io/hts-specs/VCFv4.3.pdf. Section 5 is the section you will be interested in.

In your particular example, the variant supported by your split read depends on which, if any, of your split read alignments were aligned to the negative strand and the relative read position of the split reads (i.e. which end is soft/hard clipped). Your read may well support a tandem duplication of base chr1:100-500 inclusive if the CIGAR of your primary alignment is 200S100M and your supplementary is 200M100H.