I highly recommend you get
one of these (Amazon - tongue scale) so that you don't have to guess.
You're guessing is likely close, but it's still guessing.
Also, please keep in mind that going under ~12% on tongue weight (with 13% recommended) can result in extreme uncontrollable sway. Unless you load VERY carefully your trailer will likely be around 4700-5000 lbs if you leave the water tank empty after being loaded (or more). That means a good tongue weight is more like 600 lbs for that trailer. Even at dry weight, you're looking at a safe tongue weight of ~533 lbs. Whatever your setup, you should also load up for a trip and stop off at a Cat Scale to get true weights when loaded. I do that about once a year just to make sure I'm still within limits.
This has nothing to do with it being a tandem axle trailer, by the way. My last trailer was a tandem and weighed about the same as my current trailer.
In other words: It's too much trailer for your tow vehicle and planned passenger load. I don't say that much - I'm not weight police. Look at what I tow with. IMO, one should always stay below the limits, even if it's just below. A properly setup trailer can tow nicely at reasonable speeds near the limits, but one really shouldn't go over. In this case, I don't see how you can avoid it.