What is a general rule for hitting out of these lies? 1 club more for the first and two clubs more from the second? Or do you add a percentage of the normal yardage to the total?
Every situation is different. A lot depends on how much fairway (and/or green) you have to play with and what obstacles or hills may be in the way. The higher the lie % (30-40 or 40-50 for example), the less distance your ball will carry in the air and the more it will roll once it lands.
For a "general" rule, I add about ½ of the lower lie % to my shot to start with. For example, for 30-40 rough I'll add about 15% to my shot and for 40-50 rough I'll add about 20% to my shot to start with. That usually means going up a club or two... sometimes more, sometimes less... again, it depends on the landing area and any obstacles you may have to clear.
You can also try a punch shot rather than a full shot. A punch shot will not carry as far in the air, but will roll a lot more.
Hope this helps. Play with it and see what you think.