I'm thinking 10000 coins for a masternode. 0.25 coins as reward for every block. 1 block per min. 1440x0.25=360 coins per day / 131.4k per year. (3% inflation of current supply over year).
Does the code/fork you're working on allow POS? Idealy POS and masternodes will keep as much coins off the market as possible. Maybe 0.17 coins per block. Another 2% inflation. POW also works. I don't know which works better with masternodes.

5% inflation or less in total seems reasonable.