You are at a pretty good shop if they are putting you into a drysuit for an open water class!
The first thing I do is getting all the air out of the drysuit. You need to make sure the dump valve is working right, and that you are working it right as well.
The second trick is getting all the air out of the B/C. Lifting your dump hose high and holding down the deflator button for a long time should do the trick.
The third thing is getting all the air out of your lungs. Exhaling completely, slowly, should start your descent.
A divemaster or an instructor should be able to help you set your weighting at the beginning of a dive. Then you need to adjust it yourself slightly by trial and error based on the end of the dive. You may want to take Jonnythan up on his offer. Or go to a dive store and use their pool, and get someone to help you there. I agree with Jonnythan's 2 lb increment corrections.
If you do all of that properly, and you begin to sink slowly at the beginning of your dive, then you should be close to proper weighting. If you do not sink at all, well, obviously you need more weight. If you sink really fast, they you are probably overweighted.
There is about a 25% difference between fresh water and seawater. For seawater you obviously need more.
You will not really know until the end of your dive, when you have 500 psi left in your tank, and you are hanging at 15 ft on the anchor line or kneeling in the sand underwater, if your weighting is perfect. If it is perfect, you will just slightly rise when you inhale, and just barely sink when you exhale, with your B/C totally empty, and your drysuit just comfortable, with no squeeze.
Once you get it right, you should write it into your logbook also noting the type of tank you were using. Your weighting will vary with different types of tanks. You will need more weight with aluminum tanks than with steel tanks.
If you switch between aluminum and steel, there will be a big swing of 6 to 8 lbs difference in weighting. That is the only way I can figure you would have such a big difference already. Maybe the dive store gave you a different size or type of tank on the subsequent dive?