harness lines length

Hello !

I could not adjust my seat harness to fit any higher around the waist, with the result that when, after falling in the water, I get back onto the board and I crouch to stand up , it feels like my groins and pelvis area are enveloped in a .. solid cast, which constrains my movement and takes away the necessary flexibility to easily spring up and move on. It is unconfortable and at times slightly painful. My slightly protruding abdomen, I have to admit, does not help..
I don't know if this is normal or may be I got the wrong harness size.
What this has to do with the harness lines issue will become evident in what follows.

The only way I found to make my movements easier is to hook a shoulder strap, like a..suspender, onto the harness, making sure, though, that it doesn't lift it up too much, turning it into a..chest harness. At the best I hope to have turned it into a waist harness.
The suspender may not be very fashionable, I agree, but I tried the modified harness at home and it looks I can now do the contorsions of a Chinese circus acrobat, because the harness is now high enough.

The only thing I noticed, though, is that for a standard shoulder- high boom , the harness lines now can't be above 18" , which is a far cry from the 26" or 28" many people out there suggest. To be honest with you I never understood why people should use such long lines, unless, as somebody in this forum suggested, they trip on .. salt enemas..But it is their choice, dictated by the type of sailing they do.

Any comments on these shorter 18" lines? Do perhaps shorter lines restrict the capability to lean further back/outward and to take more weight off the board ?Advantages? Disadvantages?

Thank you

Francone