Okay, here are my thoughts.
Best news I read is you have 40 good Chinese Chestnut Seedlings. That is cause for celebration. Our challenge is to save as many as possible to allow them to become established chestnut trees.
1. I suggest you divide the 40 into two groups of 20. I would move all 40 out of the rootmaker 18s and get them into root pouches now. The pouches are not expensive - but it will take more growing media than you think.
2. After the chance of the last frost / freeze, I would plant 20 of them in their final destination. They need full sun to produce chestnuts later on when they are mature enough. Don't plant in the forest as the canopy will prevent chestnuts being produced. You have got to protect those chestnuts from the critters. Two choices: either tree tubes or wire cages. Otherwise - take a good look at those 20 seedlings and get ready for them to be destroyed over time.
3. Other 20 in root pouches, I would keep at home until the fall. I would feed them with fertilizer every two weeks and stop that at July 4th. We stop at that time to allow the tender growth to harden off before the cold weather arrives. I would put a small fence around them to keep rabbits and such off them and I would water them every day or every two days depending on sun and heat. Before you put them outside you have to get them gently introduce to sunlight. Start slowing and work up to a full day over 2 weeks or so.
My recommendation splits your 40 into two groups of 20 and increases the chance that you don't lose them all.
If where you plant them you can't water - get 2 litter bottles or 5 gallon buckets to put small holes in them and sit by the seedlings to slowly water. It don't take much to save a chestnut.
Congrats on your 40 great seedlings. Best of luck turning them into established trees. It is normal to lose a few maybe 5 to 10 percent but it not normal to lose more than that unless you have sever drought.
Hope I have helped.
Wayne