How should Shinigami eyes work on Light?

I was wondering about it for kinda long time. For me - it seems, that if Light took the deal with the eyes - he would've avoided his death, because, like, he'd simply write Near's name in the notebook and just be done with any danger to his life. Yet, as it's explained, eyes shorten the remaining lifespan of a person in half. So my problem is that if Light took eyes, he would've lived longer than without them - unless he would be randomly ran over by a car even before his death in anime. Do I miss something and eyes prolonging his lifespan instead of shortening it doesn't break any rules and canon?