It is designed to run full down, that's where you'll get the most hole shot power. Once on plane, trimming up a bit will be more efficient. Without a trim gauge, look behind you at the spray coming from the sides of the drive. Trim up until that spray disappears and you'll be in the ballpark. Hint: it's not very much trim. This is the "trim" range.
There's a range of trim higher than you use for normal cruising, called the "beach range". This range is used while slow idling around shallow water and near ramps. You should not go above about 1000rpm while in this medium trim range.
The highest trim range, called "lift range" is for pulling the boat out of the water on the trailer, and the engine should never be running while the trim is this far up because it can cause damage to the u-joints. Having said that, I've forgotten to trim down a few times on the trailer before starting the engine. The u-joints rubbing on each other makes a very obvious, slightly scary noise. You'll notice when you do it.
Without a working trim gauge, it will be hard to pinpoint each of these ranges. You'll just have to go off of sound and performance until you get a feel for it.
There's a few different models of the 5.8FL that could be in your boat, so I can't point you exactly to your user's manual without knowing the full model number. But if you type in "5.8FL" in the "engine type" box it'll give you a few options down below with a free owner's manual. That manuals talks about the trim ranges.
http://www.volvopenta.com/volvopent...lication_search/Pages/publication_search.aspx
