Hello friend, well I hope not to be so radical with my answer, but thanks to the experience in my country, I consider that to obtain a minimum reward the post should meet certain requirements, (like minimum amount of words and images) if assigned such an amount is small just to participate, some people will look for the easy way and make a poor quality post just to receive the payment (since it would be insured)

I would distribute the prize like this
35 steem to the first
30 steem to the second
25 steem to the third
14 steem from 4 to 7
9 steem from 8 to 15
5 steem from 16 to 25
2 steem from 26 to 35
1 steem from 36 to 47

In this way only 3 (the three who made less effort) will not get any prize, I think it is normal that in all kinds of contests, the winners will always be much less than the participants, of course the campaign could also be approached differently , you could give a minimum amount just to make a post on the specific topic (setting some minimum parameters for the post to be accepted) in this case could be 3 steem per valid post, and only award to the first 5 places, first place 58 steem, second place 40steem, third place 30steem, fourth and fifth 20 steem