What could I do to measure block propagation time?
An estimate of block propagation time would by necessity consider the speed of a sample of nodes in the p2p network. As such it isn't possible to get an estimate using your node alone. You could measure the time it takes to upload/download blocks to/from peers as well as verify them.
To estimate propagation time you would need several nodes dispersed geographically and with unique peers. You could then monitor how long it took between for first and last node to receive the block.
You could also listen to your connected peers for their announcement of new block headers. This would allow you to estimate propagation time based on more nodes.