The only way to determine accurate range without a visual is by using active sonar. Ping. This WILL be picked up by listening hydro stations tho.
If you are very clean with taking bearings from hydro, you will get accurate range guestimates too .. with some practise and the four bearings method listed below.
You dont need to know the exact distance at this stage. You time your bearing moments. Because you know there is a fixed time between each bearing, and assuming the target has not changed speed (which they generally dont do unless being hunted) you can work out it's pretty exact course. You then plot an intersect point and work your way into visual range. As soon as you have visual, you can use the stadimeter for distance, after which you can plot a more exact course, taking range into account.
Check this post and guide for more detail and a better explanation.
http://www.subsim.com/radioroom/showthread.php?t=179137
In the upper left corner is a picture of your scope .. it has some arrows above and below it .. on for full raise/lower, one to raise/lower while pressed .. you did extend it, did you ?