I prefer them just above grate level maybe 1-2" so I'd do 1 for each shelf. I also use a few oven thermometers that I can move around on the inside if I feel like I need more information. Use ash, water, and newspaper to clean the glass on those.
I put two in mine, but it's a little bit longer that yours. I placed them between the two cooking grates so they would be as close to the food as possible. When I cook I have a space temp probe with my iGrill that I hang just under the top grate in the center of the smoker so I have a temp reference there too.
By doing this I've learned that when I have things optimal the three temps are close together...when my right temp is substantially higher than the left my fire is too hot and when it is lower I need to fire up...and add some charcoal if the wood I'm using isn't maintaining a good bed on its own.
Thanks for the reply. I also have to digital probes with remote monitoring that I plan to use, but want to have a permanently mounted thermometer for quick glance and backup purposes
I have a RF about the same size in my backyard. I used 1 T gauge at about 2" above the bottom grate which is really the main grate. I use it for getting her fired up and quick glance when I walk by. The Maverick 732 is what I really do the monitoring with from my recliner. As far as equal temps on bottom and top shelves. I don't see that happening on a small RF as the heat coming from the baffle plate opening hits the top of the CC and moves across to the stack. Yes the lower grate gets radiant heat from the BP but the top shelf seems always to be the hotter of the 2.
jm2cw
Making memories.
Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more........