Never ever use tap water! Quality water is available everywhere ... even in front of safeway; Glacier Water Vending Machines $.75 for 5 gallons! You won't need any water treatment chemicals such as Prime if you use RO water.
Tap water contains heavy metals some of which are deadly to fish, corals, and inverts. Tap also water contains tons of phosphates and often times nitrates. I don't even drink tap water anymore because of what I've learned ... not to mention it tastes like $h!t compared to the sweet taste of RO water!
As far as salt goes ... don't listen to your LFS! Sometimes a 25 gallon worth amount of salt will make more or less, and your LFS probably didn't take account for displaced water by LS and LR. Pre-Mix some salt and water into a bucket, agitate the water to aerate and dissolve salt, let sit and check your salinity. If you are using a swing arm hydrometer I highly suggest you check it against a refractometer if you can't afford one; this is what I did to ensure my swing arm was calibrated. I hope you are not using a Marineland Labs Instant Ocean Hydrometer ... they are AWFUL and off by A LOT! My LFS guy and I opened a bunch to test them and some where off by +/- .005! That can spell trouble from the begining. The Deep Six is a much better swing arm hydrometer. Anyway after you let the salt mix properly pour it into your tank ... when you are nearing the last few inches to the top of your tank take a reading from your tank with the hydrometer, this way you'll know where you're at. Then you can adjust just in case if you went over or under your salinity.
Good Luck!