It's currently random.uniform(0.0, ack_deadline * 0.9) here.

1. We should probably have some minimum so it doesn't lease too frequently.
2. The point of setting the max to ack_deadline * 0.9 is to leave enough wiggle room so the lease requests can be sent and processed by the server before the previous leases expire. We should probably make sure this time allotted to request-sending/server-processing to be some reasonable minimum (like 50 ms). No point in leaving wiggle room if the acks are going to expire by the time the new lease modacks arrive at the server.

This came up in the review of #662.