Seedling issue

Looks like you stunted it with light. Nodes never grew apart in 12 days? Even 75% may not be good if it's a light powerful enough to handle a 12/12 cycle in same area. Hard to know without the model# and being able to look up spec sheet. But read up on DLI and guesstimate what you are giving based on umol/s of light, area of coverage and hours of operation -- can easily find a table to reference these things to avoid any math. anyway, 35-40 dli is going to be the zone where there is too much light possible relative to ambient CO2 and othe renvironmental factors, which is why it's more of a grey area than a specific point because one garden can be different from another in multple related respects. try 66% from 18" or so... if that works well that makes "12/12" cycle for flower easy.. if going from 18h to 12 it's (18/12) or 150%. 1.5 x 66.7 = 100%. If it's an autoflower, you'll never want 100% power if this is the case. DLI is key, not "more light" .. ther is a daily limit to what the plant can handle DLI, daily light integral - read the wiki, get the gist. you can turn an unknown into a simple procedure and minimum trial and error with light power/hanging distance combinations that are near optimal day 1. Always requires some trial and error due to reasons explained above (relative variables that are different in each garden)