I need to come with some kind of index number for this situation:

There is an internet website with 1.000.000 online memebers. Every day (on average) 2.000 new memebers registrate. All those members ask 2.000 (on average) question (e-mail) per day. So for example in April, there were 1.000.000 and at the end of the month 1.060.000 (1.000.000 + 30 days x 2.000). So on avarage there were 1.030.000 memebers in April, and they asked 60.000 questions (30 x 2.000). Index number is defined as number of questions per month per 1.000 members. In this case it would be 60.000 devided by 1.030.000 (average number of members) multiplied by 1.000. This is 58. So far so good.

But if a month has more (or less) than 30 days, for example 31 this will happen:

Average number of members = 1.031.000 (1.000.000 + 1.062.000 / 2)

Number of questions 62.000.

Index number is in this case 60. This is because the average number of members doesn't increase with the same speed as the number of questions.

How can I come up with an index that is representative for the monthly amount of question per 1.000 members?