For public broadcasting your transaction you would need to predict when a new block is mined within seconds (and your transaction has reached the mining pool's mempool and has been properly processed to be included in the next block). This is impossible.

For slipstream.mara.com it doesn't really matter when you submit your transaction to them. If their claim is to be trusted that a submitted transaction will be mined by Mara pool non-publicly and the pool doesn't exploit your transaction you could submit it at any time (maybe 90min after Mara's last block).