The MRR calculation does not do a proration calculation. If it is a monthly subscription, then it will simply count the amount of the charge associated with the active subscription as it's normalized to months.
For example, if the subscription is opened on March 29th and has a billing period of March 29th - April 29th, it will contribute $100 each day in MRR. You can look at all charges associated with subscriptions and add. If they are daily subscriptions, you would multiple them by 30 and if they are yearly subs, you would divide by 12. The monthly interval for MRR simply takes the MRR on the last day of the month (or the most recent day if it is current month).