There's a detailed discussion about this topic on this very same page!

Typically you CANNOT just add rating points together.

You have to examine past results to produce a table converting ratings difference to relative chances - then add those.

A rating of 80 (i.e. 20 points worse than 100) is typically very bad. Yet your assumption is that an 80 horse will beat a 100 horse 80 times out of 180 i.e. ~43%.

Ridiculous!