Wow. Ra is one of my highest rated games and me and my friends have been playing it wrong the entire time. We'll have to try it the correct way. Hmmm, I really like the way we play it, it makes it so putting out a low tile, you have to be sure of the spreads worth so you dont get out bid a lose your sun tile... No idea we were playing a variant the entire time. It made calling Ra with low numbers a great tactic, as it was usually a great way to steal tiles and usually the only way to get something with low numbers.
Calling Ra with low numbers is already a great tactic. The way you've been playing it would seem to greatly reduce the effectiveness of the low tiles - removing some strategy from the game.