The correct answer here is not to go by what is written on the tire, not to figure out some mathamatical formula that is a percentage of that , but to go to the tires manufacturer and find their reccomendation for your bike.
Metzler , Dunlop, Avon, etc all have charts that list the suggested pressure for their particular tire on a particular bike.
Metzler reccomends 36 front and 42 rear I believe for their 880's on the VN750 ...42/50 is a bit too high.
The next correct answer is to spend the time testing your tire by inflating it with various pressures and then taking tempature readings after 20 minute runs at the same speed. This of course can take along time, requires some good equipment, and may result in premature wear to the tire.
But, you can use the method to fine tune whatever figure you have found that was reccomended. Although the manual does say 28/30 for the stock tire, I found 29/32 to run cooler. (This was on the Bridgestone Excedra's)
The key there is finding the "coolest" tempature. As mentioned the load on the tires has to be considered , but I would not inflate the tires to what is written on the tire , but by what the company that made the tire reccomneds for that particular motorcyle.
And as you did not tell us what kind of tire you have, only that it is the stock size, I am unable to give you an anwer as pressures differ alot from tire to tire...........