[sharg54]

I would defiantly check out the Thermometer and also mash at a higher temp. I normally mash my stout at around 156 for an hour when I single step. You can also add some more oats to bring up the mouth feel. I go with about a pound and a half on a 5 gallon batch.   

Another option, rather than trying to figure out the offset, is to just heat your liquor 5-10 degrees higher than your desired strike temp, dump it in, and wait til it drops to where you want it.  Then add the grist.

The problem I see with this idea is your going to lose temp the moment you add your grist and are going to have to add more water to bring it up again. Depending on how big your turn is it may not be a good idea. If your having problems getting your mash temps right I would recommend a good brew program like beer smith or pro mash to help you out. Also keep some boiling water and cold water on hand in case you miss the mark one way or another. And lastly I would totally change the yeast you are using. I use White Labs Irish ale 004 and have found it works best for stouts. I've tried others but just don't get the fullness of taste and feel I get with the WL yeast. Lastly take notes always and look at how your brew day went. The notes can help you isolate problems and make adjustments the next time and it has also helped me understand my equipment and how its working. Mash turn temps before you infuse, grain temps the whole nine yards. Ive found for instance if I preheat my mash turn to 72 degrees before I start, my temps stay more stable.  Good luck and happy brewing.

[Hokerer]

Another option, rather than trying to figure out the offset, is to just heat your liquor 5-10 degrees higher than your desired strike temp, dump it in, and wait til it drops to where you want it.  Then add the grist.

The problem I see with this idea is your going to lose temp the moment you add your grist and are going to have to add more water to bring it up again. Depending on how big your turn is it may not be a good idea

You misunderstood what I was saying.  When I said "where you want it", I meant the strike temp that you want (or your software calculated for you).  Strike temp is higher than your desired mash temp by the amount "you're going to lose the moment you add your grist".  So, by adding the water higher than strike and letting it drop, you're eliminating the thermal mass of your equipment from the equation.  Since thermal mass is the most unique and difficult to dial in parameter, eliminating the need to figure it out is "pragmatic"

[jaybeerman]

When I said "where you want it", I meant the strike temp that you want (or your software calculated for you).  Strike temp is higher than your desired mash temp by the amount "you're going to lose the moment you add your grist".  So, by adding the water higher than strike and letting it drop, you're eliminating the thermal mass of your equipment from the equation.  Since thermal mass is the most unique and difficult to dial in parameter, eliminating the need to figure it out is "pragmatic"

+1 this is the only way to do it

[Pi]

Since I am using a RIMS, i dough in a couple degerss lower (Celsius) and come up to rest. Reading this thread raises a couple questions i have:
Since strike water is usually pretty close to mashout temperature is there some denaturing going on when I dough in/mixing?
And, if you rest at a higher temp. for a long period will you get less body? I am always curious when i look at other recipies how much mash times/temps vary with the same style.Can anyone recommend some reading that helps "demystify" mashing?

[wildknight]

I used the London strain with my English Mild and that stuff cranked pretty good.  It turned out a couple points lower than expected and the perception was lighter than the gravity reading.  You might try a less attenuative English strain. 

Just to some food for thought, but adding oatmeal to a 1.050-1.060 stout isn't gonna give it the same body as a 1.100-1.110 Russian imperial stout.  Try the good tips others have offered, but keep your expectations within reason.  You could always buy a bottle of Samuel Smiths and taste them side by side; you may not be as far off in mouthfeel as you think.