yeah I would also say to try to get a feel for the weight when they are watered and when they are not watered. Thats what I did before I got a moisture meter. Also if the top is still moist then no need to water.
Also I came up with a good cheap moisture indicator for your soil. Buy some wooden shish kabob stakes(sorry for spelling) and poke it into the soil
the wood will come back wet if it is moist or remain dry if the soil is dry. Make sense? Ok welll I hope this all helps. I have like 38 seedlings right now. they are about the same size as yours. How are you watering and what are you using as far as water? Example tap or bottled? I use purified bottled water and I spray my soil with a spray bottle set to mist........
good luck and keep us posted.