Sure, it takes 33 feet of saltwater to make 1 atm of pressure, and 34 feet of fresh... so basically, you are one foot off for every 33 feet of depth...
So 3 feet off at 100 feet... pretty insignificant - so most people just ignore it.
In case you cared, the reason is because salt water is more dense due to the dissolved salts, so a cubic foot of fresh water weighs approx 62.4 lbs, while a cubic foot of salt water weights 64 lbs.
This is also the reason you need to add about 5 lbs of lead when moving from fresh to salt to maintain your buoyancy.
Quick edit: your answer is 59.2... just take your ffw depth divided by 34, then multiply by 33... Make sense?