Can MahjongTime Rating System be Improved?

We have received inquires from members of our website regarding our ranking formula. The formula is based on Official International rules:

RatingNew= RatingOld+(Score+Difficulty-RatingOld)/41

Gamefactor = 40 on the gamefactor is static -- this is a number to average
out the luck factor of the game

Difficulty = average of all the participating players' ratings.

There are a few questions that we are getting from members:
1. Should we decrease the ratings for users that do not play for a while on the website?
2. Currently if at least 2 real players are on the table the rating is calculated. Should we change the way the rating is calculated in case when users leave the table?
What can be done to improve our rating system? Please post your opinions below.