Masternode rewards are based on how many coins are locked in masternodes.
There is a hard limit of 60% of the block reward.

Here are some rough examples of block rewards;

25% Supply locked = 55% to masternode and 45% to miner
40% Supply locked = 40% to masternode and 60% to miner
55% Supply locked = 25% to masternode and 75% to miner