I was thinking of tying it to plutocratic, because most "tall" states are probably either republics (venice or milan for example) or england. And england can also change into a republic iirc (rep. dictatorship under cromwell; if I am not mistaken).
I specifically did not tie it to economic, because you alredy get reduced development from that idea. (I don't like, that economic is the only way to reduce development cost apart from universitys.)
Also I feel that "innovative" kind of fits the idea of a tall state. And don't forget with reduced tech cost and cheaper advisors innovative has great potential of giving you a mana surplus, which you will most likely use for development.
As for administrative: That idea gives you reduced core creation already, so helps you to grow wide. I am not sure if it should get a bonus to tall development at the same time.
As for how much reduction the policy should give... Most policies are about as strong as 1-2 ideas.
One eco idea gives 20% development cost, but (almost) all NIs only give 10% development cost (exceptions are prussia with only 5% and lan na with 20%)
So I think a 15% reduction would be fine. After all that policy would also reduce advisor cost at the same time.
Edit: My second choice would have been innovative + trade. But that policy already is very unique. It's the only policy for reduced interest. Of course it could become an interest & development policy.