Another trick worth considering - If you can suit up in an almost identical set of gear and hit the pool back home (like if you'll be bringing your own wetsuit for instance), see if you can get in some pool time. Determine your freshwater weighting with a similar (empty) BC and a similar (nearly empty) aluminum tank. Basically you're figuring out how much weight you need to be neutral in fresh water in a very similar configuration. Then dry everything off, step on a scale, fully kitted out including the empty tank and any needed lead. Saltwater is 2.5% denser than freshwater. For example, maybe yourself, your kit and a couple pounds of lead adds up to like 240 pounds total. 240 * .025 = 6 pounds. In this example, you would need to add 6 more pounds of lead for the same configuration in saltwater as you would in freshwater.
The operator might be able to share what gear they're supplying, and you might find the exact same stuff for rent/borrow back home.
A few years back, when I decided to travel with a 3mm and a AL backplate, the math above turned out to be accurate within a pound of neutral when I finally got a chance to do a good weight check with a near empty AL80.