Sandy, Leaving out of Lake Worth Inlet, it's 54 nautical miles to the buoy at West End. You can fuel right at the Inlet at Sailfish Marina by Peanut Island to top off.
With my 90 Optimax, at cruise in blue water I run 24-25 mph burning right at 5 gallons per hour, plus a 20% fudge-factor. So I figure 10 gallons over, + 2 gallons fudge-factor = 12 gallons or <1/2 tank.
Even when I had the Mercury XR4 150, at 25 mph in blue water I burned only 7 gallons per hour or 3.6 mpg; so 14 gallons over + 3 gallon fudge factor = 17 gallons / 60% of a tank.
I haven't crossed since changing from the 150 to the 90 hp, but I have run 88 miles out into the Gulf of Mexico to the Florida Middle Grounds. I carried 18 gallons extra fuel last time I went, and burned it first. No trolling, two drifts, and then anchored up. GPS said total distance travelled was 191 miles round trip, and filling my main tank upon my return showed I had 8 gallons useable fuel left upon my return, which is more than the 6 gallon/1 hr reserve I planned upon.
Going over to the Bahamas in July, it's best to leave early AM,
BEFORE Sunrise, while it's glassy. I've twice made the trip in my Seafari running 33-34 mph all the way over, which was my best fuel economy speed with the XR4 150. (7.9 gph or 4.2 mpg)
While I haven't made the run to the bahamas in nearly 8 years, I used to run over 3 to 5 times per week. (Yes, per week) I normally would sccot over after work, make a couple of dives, and come back same night. Mostly, I made the trip in a 25 Robalo with twin 200 mercs that belonged to the marina, and took the marina owner with me. (Free fuel that way) Also, afternoon weather and seas weren't as much of an issue with the bigger boat. It generally picks up to 2'-4' offshore in the GulfStream by 11 a.m. or so because of the seabreeze.
I try to time my departure for arriving in West End before 9 a.m. I allowed up to 3 hours for the crossing, steering a course of 104*. If I crossed in 2 hours, the tower at West End would be 1 mile north of steered course. If I took 3 hours, it would be 1.5 miles south of steered course. Even if I took 4 hours, the tank would be visible just 4 miles south of steered course. GulfStream current runs about 2.6 kts in the early summer, but varies from 2.3 to 3.0 kts in places. Coming back, steer 255*, and you'll see the Tiara condo which is 1/2 mile N of Lake Worth Inlet from 15 miles offshore.
But of course, with GPS nobody really navigates anymore.