Nice little space, remember you don’t want any light leaks or lights inside your tent (think LED’s on power bars or things) otherwise the photos will be unhappy and eventually herm.
You have a window there, you will need to create a vent out of it. The AC window vents work well but if you have to you can make your own that’s better and weatherproof with a piece of foam board, some weather stripping and a vent ducting flap.
This is because you want to exhaust your spent air, plants really really want lots of carbon dioxide and right now your loft is oxygen rich which will hurt your plants everytime they circulate that air.
Here is mine, I have two tents, many use wood or the pink/blue solid foam board but this worked for me in a pinch and holds up at - 40 in the winter so I’m leaving it alone lol
As for the lighting a good rule to follow is as your plants grow they have node spacing, which is where a set of branches/leafs come off the main stock, keeping that node spacing at about 1/2 inch is considered a good rule of thumb.
Height is a terrible measurement to use for lights. With an app you can quickly dial in your DLI Wich is a much better approximation of your lighting not only to you but also to share with others it’s more of a universal power rating over 24hr
Here is a thread I created that has evolved so read along to the end and other have suggested a few new apps on your phone that tend to work, don’t be scared by the math at the start.