Done a few tests (only done half the teams):

Chiefs vs Highlanders:
Mine: 30-24,
Real: 19-7,
Spread was Chiefs +4.5.

Crusaders vs Blues:
Mine: 28-25,
Real: 29-38,
Spread was Crusaders -2.5.

Crusaders vs NSW:
Mine: 31-23,
Real: 19-42,
Spread was Crusaders -8.5.

Blues vs Chiefs:
Mine: 29-23,
Real: 20-27,
Spread was Blues -18.5.

Hurricanes vs Blues:
Mine: 25-26,
Real: 26-26,
Spread was Hurricanes +7.

If I had bet these I'd be 4-1, although you probally wouldn't bet NSW +8.5 if you thought the line should be 8. Only a small sample so I'll see how it does against another 5-10 games.