I really don't have a good answer for that one, fanfan. Think what I would do is try to group the seedlings into 2 heights and put them under the light for 12-hour shifts, and adjust the light for the other group. Might be able to do three 8-hours. You can always keep the shorter ones around when lighting the taller group(s).
Another thing you could do - put a 2x6 or two under the short seedlings to bring them up to the height of the taller seedlings perhaps?
Today is the first day of all day sun in our new digs. I am still trying to decide on how to orient my new garden. It looks like which ever orientation I decide to take that the garden would get full morning sun until around 1 to 2 PM and then be shaded the rest of the afternoon. At least at this time of year. The shade might be a blessing this summer?