They are not always that accurate or as useful as you would like them to be.
Start by counting the number of 'clicks' of the trim out to go from all the way IN, to all the way OUT. On my Bravo III, if I recall it was about 6 clicks. Remember we're talking about the clicks to the Trim Limit, not all the way out with the trailer button.
Once you're on plane, trim out (counting each click) until you hear the prop start to cavitate, then click in (again counting each click) until it stops (Learn to keep track of the number in your head.) That is your ideal trim angle. If it gets rough and you hear the occasional cavitation going down a wave, then click in again until that stops.
That process will be way more accurate than a trim guage.