Update user.class.php

I have noticed it is possible to put in extremely low values for the donation percent, values like:  0.000000000000001.

According to my math, even if I mined every single dogecoin, such donation would work out to:
100,000,000,000 * 0.00000000000000001 (0.000000000000001%) = 0.000001 which is less than the current minimum transaction amount of 1.

Would it be better to make the minimum a more sane value such as the proposed 0.01%? With this value a reward of 10,000 coins would be a modest donation of 1.

I am not against using an even lower threshold, such as 0.001, but think some sort of adjustment should be in play to keep the value reasonable. 

My main concern, and I am no expert, is that allowing such extremes would place extra load on the backend?