Bonus equivalent for rerolls?

[Flobby]

Doing some experimenting here....

I remember reading somewhere that rerolling a d20 is roughly equivalent to a +2 bonus (if I remember correctly).

What if I use 2d10 and instead of a reroll, roll 3d10 and take the best 2? Does anyone know how to figure out what the equivalent bonus would be?
Does my question make sense?

(For example, best 2 out of 3d10 is equal, on average, to 2d10 + 4)

 

[Nagol]

It makes the curve more pronounced and shifts the mode up 3 points.

Here are the raw stats:

    Roll       %-2d10    %-3d10 choose 2
         2          1           0.1000
         3          2           0.3000
         4          3           0.7000
         5          4           1.2000
         6          5           1.9000
         7          6           2.7000
         8          7           3.7000
         9          8           4.8000
        10          9           6.1000
        11         10           7.5000
        12          9           8.8000
        13          8           9.6000
        14          7          10.0000
        15          6           9.9000
        16          5           9.4000
        17          4           8.4000
        18          3           7.0000
        19          2           5.1000
        20          1           2.8000