Place a thermometer neck to the tray, underneath the dome or saran wrap. You can split unwanted seedlings into 3 groups. I ve been very successfully cultivating for decades and I ve always just placed my seeds, be it 1 or 100, in between 2 folded sheets of tissue which I ve made wet with tap water.  Source: <a href=https://halin.pl/how-long-do-weed-seeds-stay-good/>https://halin.pl/how-long-do-weed-seeds-stay-good/</a>