Say I am looking to run a farmer strat. I decide to stay m and tech early. Say I tech at 3k acres and tech out to 228% at 72k agri tech. Do I need to tech more as I then grab out to 20k acres will my agri tech % still be 228% at 20k acres??

Tech lvls will decrease as you get more acres.

You need more tech for more acres.
Maximum tech % does not change but the number of tech require to achieve that will increase as your acres increase.

You do not want to tech enough at 3k for you to maintain all the way until 20k. If you tech start just tech enough so that you are good until you are doubled the land. You should buy tech as you go

there's a pretty accurate tech calculator on http://www.eestats.com

You need the same "X tech points per acre" ratio of tech points to your total land, in order to stay at the same tech%.