Oh right I forgot your using led
you half the wattage needed so for a 50sq inch screen needs 25w of led
so you should be good in that department and can have a 100sq inch screen, the main thing is to get all the screen lit up good.
For lighting times you should light it for 18 hours and run it opposite your tank lights then you won't get ph swings !
Looks good by the way and keep us updated on algae growth, eventually it will grow algae on the screen and not in your tank
For nutrients issues it can take upto 8 weeks and some really good lighted scrubbers this can take 4 weeks.
Just remember no matter what clean the screen every week, people leave them growing algae longer because it not "thick" enough, this is bad as the algae will cover itself and shade the algae underneath.
Hope this helps mate !