When Rain Man couldn't guess correctly

You mean with the wheel of fortune? I don't think so. I think that scene was to remind the audience that Charlie doesn't completely understand how Raymond's mind works. Raymond may be very good with mathematics, but he also couldn't do a simple calculation of one dollar minus 50 cents. Likewise, his mathematical skill doesn't necessarily mean he can predict where a spinning wheel will stop.

Throwing a dart at a target does not have math involved with it, so he was expecting too much from Raymond to be able to correctly guess at a task that has a completely random probability. This was the first instance of Charlie feeling bad for depending on Raymond to earn money.

The scene was brilliant to remind the audience while blackjack can be beaten by a skilled individual, you cant beat wheel of fortune because its 100% game of chance.