For FWTO it's number of votes received by the winner divided by the total votes cast. Each voter casts 6 votes, so say 16 of the 18 voted (I don't know in this case), then that's 26 / (16*6) which would be about 27%. So if everyone who voted gave you 3 points then you'd get somewhere around 45-48% depending on how many entered and how many voted, it will vary from round to round. So a score in the high forties will reflect the maximum possible score.
FWPT is different, the percentages reflect the percentage of the maximum possible, so if everyone who votes gives you their 3 points then you get 100%.