MIN and MAX set the number of seconds out of 100 for each. In other words, If FR is 0, the stoker will be run at the MIN rate you have set. If MIN is 6 that means it will be on for 6 out of every 100 seconds. If FR is at 99 then the stoker will be run at MAX. If MAX is 40 then it will be on for 40 of every 100 seconds.
FR is the percentage of the range defined by the MIN and MAX settings. A MIN of 10 and a MAX of 20 and a FR of 50 will yield a stoker on/off mix of 15 seconds on and 85 seconds off.
A MIN of 10 does represent twice as much on time as a MIN of 5, at least at FR 0.
To relate all of this to amount of coal burned... Figure 13KBTU per pound or rice coal. A burner rated at 90KBTU input at max burn rate, a MIN of 6 and a MAX of 40... MAX burn rate is 6.9 lbs/hr, MIN is 1.04lbs/hr. In your case of a MIN of 5, that's roughly .87lbs/hr. These may be a bit different if a different KBTU per pound figure was used in the rating of the stoker's input BTU rating, or if you're running a different MAX setting.