Hi Michael
Long time a go I send a formula that I use to calculate equivalence between performances in lake and sea take a look at:
http://forums.deeperblue.net/showthread.php?s=&threadid=19013
The main differences are:
Cold= Usually lakes are colder than sea, but not in all places, so you must take into account wetsuit width (or no wetsuit)
Density= As you know fresh water has less density than salt water, so you will find that it's easier the descend phase of the dive, and you will feel less propulsion on the ascend, so you have to balance your ballast
Altitude= Depending on the altitude the Inspired Fraction of Oxygen is less than at sea level, no problem if the lake is at sea level altitude
Visibility= At least in our lakes, visibility is awful, you should pay more attention to security and probably need to use some kind of ilumination.
My advice is that don't extrapolate, just take it easy at the beggining and progress in depht as you feel comfortable