What is the Optimal block size based on 90 percentile block propogation delay?
I think block propagation delay is the only limiting factor in increasing block size.

This assumption is not accurate. While block propagation time is an important parameter for fair competition among miners, the block size also affects initial synchronization times of new nodes, disk space for archival nodes, and bandwidth (especially upload) requirements for all node operators.
Block propagation between miners should preferably take less than a second, but I don't think the block size and propagation are as related as the question surmises and thus consider the main question to be unanswerable.